diff --git a/09_dynamic_programming/ruby/01_longest_common_subsequence.rb b/09_dynamic_programming/ruby/01_longest_common_subsequence.rb
new file mode 100644
index 0000000..624023e
--- /dev/null
+++ b/09_dynamic_programming/ruby/01_longest_common_subsequence.rb
@@ -0,0 +1,7 @@
+if word_a[i] == word_b[j]
+  # The letters match.
+  cell[i][j] = cell[i-1][j-1] + 1
+else
+  # The letters don't match.
+  cell[i][j] = [cell[i-1][j], cell[i][j-1]].max
+end