diff --git a/tests/test_pattern_merger.py b/tests/test_pattern_merger.py
new file mode 100644
index 0000000..aa0461a
--- /dev/null
+++ b/tests/test_pattern_merger.py
@@ -0,0 +1,116 @@
+"""Tests for pattern merger module."""
+
+import pytest
+
+from gitignore_cli.pattern_merger import PatternMerger, merge_templates, deduplicate_patterns
+
+
+class TestPatternMerger:
+    """Tests for PatternMerger class."""
+
+    def setup_method(self):
+        """Set up test fixtures."""
+        self.merger = PatternMerger()
+
+    def test_merge_single_pattern_set(self):
+        """Test merging a single pattern set."""
+        patterns = ["__pycache__/\n*.pyc"]
+        result = self.merger.merge_patterns(patterns)
+
+        assert "__pycache__/" in result
+        assert "*.pyc" in result
+
+    def test_merge_multiple_pattern_sets(self):
+        """Test merging multiple pattern sets."""
+        patterns = [
+            "__pycache__/\n*.pyc",
+            "node_modules/\n*.log",
+        ]
+        result = self.merger.merge_patterns(patterns)
+
+        assert "__pycache__/" in result
+        assert "*.pyc" in result
+        assert "node_modules/" in result
+        assert "*.log" in result
+
+    def test_deduplication(self):
+        """Test that duplicate patterns are removed."""
+        patterns = [
+            "*.pyc",
+            "*.pyc",
+            "*.pyc",
+        ]
+        result = self.merger.merge_patterns(patterns)
+
+        assert result.count("*.pyc") == 1
+
+    def test_empty_patterns(self):
+        """Test merging empty pattern sets."""
+        patterns = []
+        result = self.merger.merge_patterns(patterns)
+
+        assert result == "\n"
+
+    def test_whitespace_lines_removed(self):
+        """Test that empty/whitespace lines are handled."""
+        patterns = [
+            "*.pyc\n\n\n  \n*.pyo",
+        ]
+        result = self.merger.merge_patterns(patterns)
+
+        assert "*.pyc" in result
+        assert "*.pyo" in result
+
+    def test_normalize_pattern(self):
+        """Test pattern normalization."""
+        assert self.merger._normalize_pattern("*.pyc") == "*.pyc"
+        assert self.merger._normalize_pattern("  *.pyc  ") == "*.pyc"
+        assert self.merger._normalize_pattern("") == ""
+        assert self.merger._normalize_pattern("# comment") == ""
+        assert self.merger._normalize_pattern("!important") == "!important"
+
+    def test_deduplicate_patterns_function(self):
+        """Test the deduplicate_patterns convenience function."""
+        patterns = "*.pyc\n*.pyc\n*.pyo"
+        result = deduplicate_patterns(patterns)
+
+        assert result.count("*.pyc") == 1
+
+    def test_filter_comments(self):
+        """Test filtering out comments."""
+        patterns = "*.pyc\n# another"
+        result = self.merger.filter_comments(patterns)
+
+        assert "*.pyc" in result
+
+    def test_preserve_negation_patterns(self):
+        """Test that negation patterns (!) are preserved."""
+        patterns = [
+            "*.log\n!important.log",
+        ]
+        result = self.merger.merge_patterns(patterns)
+
+        assert "*.log" in result
+        assert "!important.log" in result
+
+    def test_empty_string_patterns(self):
+        """Test handling of empty string patterns."""
+        patterns = [""]
+        result = self.merger.merge_patterns(patterns)
+
+        assert result == "\n"
+
+
+class TestMergeTemplatesFunction:
+    """Tests for the merge_templates convenience function."""
+
+    def test_merge_templates_convenience(self):
+        """Test the merge_templates convenience function."""
+        patterns = [
+            "*.pyc",
+            "*.pyo",
+        ]
+        result = merge_templates(patterns)
+
+        assert "*.pyc" in result
+        assert "*.pyo" in result