diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3dc40f8a..d35f4649 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,7 +9,8 @@ and this project adheres to [Semantic Versioning](https://semver.org).
 
 ### Added
 
-- Implementation of the ISREF() information function.
+- Implementation of the UNIQUE() Lookup/Reference (array) function
+- Implementation of the ISREF() Information function.
 - Added support for reading "formatted" numeric values from Csv files; although default behaviour of reading these values as strings is preserved.
 
   (i.e a value of "12,345.67" can be read as numeric `1235.67`, not simply as a string `"12,345.67"`, if the `castFormattedNumberToNumeric()` setting is enabled.
diff --git a/src/PhpSpreadsheet/Calculation/Calculation.php b/src/PhpSpreadsheet/Calculation/Calculation.php
index 36adef01..b3988260 100644
--- a/src/PhpSpreadsheet/Calculation/Calculation.php
+++ b/src/PhpSpreadsheet/Calculation/Calculation.php
@@ -2583,7 +2583,7 @@ class Calculation
         ],
         'UNIQUE' => [
             'category' => Category::CATEGORY_LOOKUP_AND_REFERENCE,
-            'functionCall' => [Functions::class, 'DUMMY'],
+            'functionCall' => [LookupRef\Unique::class, 'unique'],
             'argumentCount' => '1+',
         ],
         'UPPER' => [
diff --git a/src/PhpSpreadsheet/Calculation/Information/ExcelError.php b/src/PhpSpreadsheet/Calculation/Information/ExcelError.php
index 98242eb6..e0f40848 100644
--- a/src/PhpSpreadsheet/Calculation/Information/ExcelError.php
+++ b/src/PhpSpreadsheet/Calculation/Information/ExcelError.php
@@ -127,10 +127,20 @@ class ExcelError
     /**
      * DIV0.
      *
-     * @return string #Not Yet Implemented
+     * @return string #DIV/0!
      */
     public static function DIV0()
     {
         return self::$errorCodes['divisionbyzero'];
     }
+
+    /**
+     * CALC.
+     *
+     * @return string #Not Yet Implemented
+     */
+    public static function CALC()
+    {
+        return '#CALC!';
+    }
 }
diff --git a/src/PhpSpreadsheet/Calculation/LookupRef/Unique.php b/src/PhpSpreadsheet/Calculation/LookupRef/Unique.php
new file mode 100644
index 00000000..67b911aa
--- /dev/null
+++ b/src/PhpSpreadsheet/Calculation/LookupRef/Unique.php
@@ -0,0 +1,141 @@
+<?php
+
+namespace PhpOffice\PhpSpreadsheet\Calculation\LookupRef;
+
+use PhpOffice\PhpSpreadsheet\Calculation\Functions;
+use PhpOffice\PhpSpreadsheet\Calculation\Information\ExcelError;
+use PhpOffice\PhpSpreadsheet\Shared\StringHelper;
+
+class Unique
+{
+    /**
+     * UNIQUE
+     * The UNIQUE function searches for value either from a one-row or one-column range or from an array.
+     *
+     * @param mixed $lookupVector The range of cells being searched
+     * @param mixed $byColumn Whether the uniqueness should be determined by row (the default) or by column
+     * @param mixed $exactlyOnce Whether the function should return only entries that occur just once in the list
+     *
+     * @return mixed The unique values from the search range
+     */
+    public static function unique($lookupVector, $byColumn = false, $exactlyOnce = false)
+    {
+        if (!is_array($lookupVector)) {
+            // Scalars are always returned "as is"
+            return $lookupVector;
+        }
+
+        $byColumn = (bool) $byColumn;
+        $exactlyOnce = (bool) $exactlyOnce;
+
+        return ($byColumn === true)
+                ? self::uniqueByColumn($lookupVector, $exactlyOnce)
+                : self::uniqueByRow($lookupVector, $exactlyOnce);
+    }
+
+    /**
+     * @return mixed
+     */
+    private static function uniqueByRow(array $lookupVector, bool $exactlyOnce)
+    {
+        // When not $byColumn, we count whole rows or values, not individual values
+        //      so implode each row into a single string value
+        array_walk(
+            $lookupVector,
+            function (array &$value): void {
+                $value = implode(chr(0x00), $value);
+            }
+        );
+
+        $result = self::countValuesCaseInsensitive($lookupVector);
+
+        if ($exactlyOnce === true) {
+            $result = self::exactlyOnceFilter($result);
+        }
+
+        if (count($result) === 0) {
+            return ExcelError::CALC();
+        }
+
+        $result = array_keys($result);
+
+        // restore rows from their strings
+        array_walk(
+            $result,
+            function (string &$value): void {
+                $value = explode(chr(0x00), $value);
+            }
+        );
+
+        return (count($result) === 1) ? array_pop($result) : $result;
+    }
+
+    /**
+     * @return mixed
+     */
+    private static function uniqueByColumn(array $lookupVector, bool $exactlyOnce)
+    {
+        $flattenedLookupVector = Functions::flattenArray($lookupVector);
+
+        if (count($lookupVector, COUNT_RECURSIVE) > count($flattenedLookupVector, COUNT_RECURSIVE) + 1) {
+            // We're looking at a full column check (multiple rows)
+            $transpose = Matrix::transpose($lookupVector);
+            $result = self::uniqueByRow($transpose, $exactlyOnce);
+
+            return (is_array($result)) ? Matrix::transpose($result) : $result;
+        }
+
+        $result = self::countValuesCaseInsensitive($flattenedLookupVector);
+
+        if ($exactlyOnce === true) {
+            $result = self::exactlyOnceFilter($result);
+        }
+
+        if (count($result) === 0) {
+            return ExcelError::CALC();
+        }
+
+        $result = array_keys($result);
+
+        return $result;
+    }
+
+    private static function countValuesCaseInsensitive(array $caseSensitiveLookupValues): array
+    {
+        $caseInsensitiveCounts = array_count_values(
+            array_map(
+                function (string $value) {
+                    return StringHelper::strToUpper($value);
+                },
+                $caseSensitiveLookupValues
+            )
+        );
+
+        $caseSensitiveCounts = [];
+        foreach ($caseInsensitiveCounts as $caseInsensitiveKey => $count) {
+            if (is_numeric($caseInsensitiveKey)) {
+                $caseSensitiveCounts[$caseInsensitiveKey] = $count;
+            } else {
+                foreach ($caseSensitiveLookupValues as $caseSensitiveValue) {
+                    if ($caseInsensitiveKey === StringHelper::strToUpper($caseSensitiveValue)) {
+                        $caseSensitiveCounts[$caseSensitiveValue] = $count;
+
+                        break;
+                    }
+                }
+            }
+        }
+
+        return $caseSensitiveCounts;
+    }
+
+    private static function exactlyOnceFilter(array $values): array
+    {
+        return array_filter(
+            $values,
+            function ($value) {
+                return $value === 1;
+            }
+        );
+    }
+}
diff --git a/tests/PhpSpreadsheetTests/Calculation/Functions/LookupRef/UniqueTest.php b/tests/PhpSpreadsheetTests/Calculation/Functions/LookupRef/UniqueTest.php
new file mode 100644
index 00000000..c0a6f88e
--- /dev/null
+++ b/tests/PhpSpreadsheetTests/Calculation/Functions/LookupRef/UniqueTest.php
@@ -0,0 +1,157 @@
+<?php
+
+namespace PhpOffice\PhpSpreadsheetTests\Calculation\Functions\LookupRef;
+
+use PhpOffice\PhpSpreadsheet\Calculation\Information\ExcelError;
+use PhpOffice\PhpSpreadsheet\Calculation\LookupRef;
+use PHPUnit\Framework\TestCase;
+
+class UniqueTest extends TestCase
+{
+    /**
+     * @dataProvider uniqueTestProvider
+     */
+    public function testUnique(array $expectedResult, ...$args): void
+    {
+        $result = LookupRef\Unique::unique(...$args);
+        self::assertEquals($expectedResult, $result);
+    }
+
+    public function testUniqueException(): void
+    {
+        $rowLookupData = [
+            ['Andrew', 'Brown'],
+            ['Betty', 'Johnson'],
+            ['Betty', 'Johnson'],
+            ['Andrew', 'Brown'],
+            ['David', 'White'],
+            ['Andrew', 'Brown'],
+            ['David', 'White'],
+        ];
+
+        $columnLookupData = [
+            ['PHP', 'Rocks', 'php', 'rocks'],
+        ];
+
+        $result = LookupRef\Unique::unique($rowLookupData, false, true);
+        self::assertEquals(ExcelError::CALC(), $result);
+
+        $result = LookupRef\Unique::unique($columnLookupData, true, true);
+        self::assertEquals(ExcelError::CALC(), $result);
+    }
+
+    public function testUniqueWithScalar(): void
+    {
+        $lookupData = 123;
+
+        $result = LookupRef\Unique::unique($lookupData);
+        self::assertSame($lookupData, $result);
+    }
+
+    public function uniqueTestProvider(): array
+    {
+        return [
+            [
+                [['Red'], ['Green'], ['Blue'], ['Orange']],
+                [
+                    ['Red'],
+                    ['Green'],
+                    ['Green'],
+                    ['Blue'],
+                    ['Blue'],
+                    ['Orange'],
+                    ['Green'],
+                    ['Blue'],
+                    ['Red'],
+                ],
+            ],
+            [
+                [['Red'], ['Green'], ['Blue'], ['Orange']],
+                [
+                    ['Red'],
+                    ['Green'],
+                    ['GrEEn'],
+                    ['Blue'],
+                    ['BLUE'],
+                    ['Orange'],
+                    ['GReeN'],
+                    ['blue'],
+                    ['RED'],
+                ],
+            ],
+            [
+                ['Orange'],
+                [
+                    ['Red'],
+                    ['Green'],
+                    ['Green'],
+                    ['Blue'],
+                    ['Blue'],
+                    ['Orange'],
+                    ['Green'],
+                    ['Blue'],
+                    ['Red'],
+                ],
+                false,
+                true,
+            ],
+            [
+                ['Andrew', 'Betty', 'Robert', 'David'],
+                [['Andrew', 'Betty', 'Robert', 'Andrew', 'Betty', 'Robert', 'David', 'Andrew']],
+                true,
+            ],
+            [
+                ['David'],
+                [['Andrew', 'Betty', 'Robert', 'Andrew', 'Betty', 'Robert', 'David', 'Andrew']],
+                true,
+                true,
+            ],
+            [
+                [1, 1, 2, 2, 3],
+                [[1, 1, 2, 2, 3]],
+            ],
+            [
+                [1, 2, 3],
+                [[1, 1, 2, 2, 3]],
+                true,
+            ],
+            [
+                [
+                    [1, 1, 2, 3],
+                    [1, 2, 2, 3],
+                ],
+                [
+                    [1, 1, 2, 2, 3],
+                    [1, 2, 2, 2, 3],
+                ],
+                true,
+            ],
+            [
+                [
+                    ['Andrew', 'Brown'],
+                    ['Betty', 'Johnson'],
+                    ['David', 'White'],
+                ],
+                [
+                    ['Andrew', 'Brown'],
+                    ['Betty', 'Johnson'],
+                    ['Betty', 'Johnson'],
+                    ['Andrew', 'Brown'],
+                    ['David', 'White'],
+                    ['Andrew', 'Brown'],
+                    ['David', 'White'],
+                ],
+            ],
+            [
+                [[1.2], [2.1], [2.2], [3.0]],
+                [
+                    [1.2],
+                    [1.2],
+                    [2.1],
+                    [2.2],
+                    [3.0],
+                ],
+            ],
+        ];
+    }
+}