diff --git a/lib/uri/generic.rb b/lib/uri/generic.rb
index 634da49..18ca589 100644
--- a/lib/uri/generic.rb
+++ b/lib/uri/generic.rb
@@ -1108,10 +1108,13 @@ def merge!(oth)
     #
     # +oth+::
     #    URI or String
+    # +allow_relative+::
+    #    Boolean (false by default)
     #
     # == Description
     #
-    # Merges two URIs.
+    # Merges two URIs. If +allow_relative+ is true, allows merging
+    # a relative base URI with a relative URI.
     #
     # == Usage
     #
@@ -1121,7 +1124,7 @@ def merge!(oth)
     #   uri.merge("/main.rbx?page=1")
     #   # => "http://my.example.com/main.rbx?page=1"
     #
-    def merge(oth)
+    def merge(oth, allow_relative = false)
       rel = parser.__send__(:convert_to_uri, oth)
 
       if rel.absolute?
@@ -1130,7 +1133,7 @@ def merge(oth)
         return rel
       end
 
-      unless self.absolute?
+      unless self.absolute? || allow_relative
         raise BadURIError, "both URI are relative"
       end
 
diff --git a/test/uri/test_generic.rb b/test/uri/test_generic.rb
index 94eea71..5fd7c5b 100644
--- a/test/uri/test_generic.rb
+++ b/test/uri/test_generic.rb
@@ -288,6 +288,26 @@ def test_merge_authority
     assert_equal(u0, u1)
   end
 
+  def test_merge_allow_relative
+    # When both URIs are relative, merge should raise unless allow_relative is true:
+    base = URI.parse('a/b/c')
+    assert_raise(URI::BadURIError) do
+      base.merge('d/e')
+    end
+
+    # With allow_relative=true, relative base + relative ref should merge paths:
+    merged = base.merge('d/e', true)
+    assert_equal('a/b/d/e', merged.to_s)
+
+    # Parent directory traversals should be handled as with absolute bases:
+    merged_up = base.merge('../x', true)
+    assert_equal('a/x', merged_up.to_s)
+
+    # Leading slash should reset to root-like behavior within relative context:
+    merged_abs = base.merge('/z', true)
+    assert_equal('/z', merged_abs.to_s)
+  end
+
   def test_route
     url = URI.parse('http://hoge/a.html').route_to('http://hoge/b.html')
     assert_equal('b.html', url.to_s)