diff --git a/lib/ipaddr.rb b/lib/ipaddr.rb
index 51c6898..e6a888b 100644
--- a/lib/ipaddr.rb
+++ b/lib/ipaddr.rb
@@ -717,6 +717,7 @@ def initialize(addr = '::', family = Socket::AF_UNSPEC)
     if prefix =~ /\A\[(.*)\]\z/i
       prefix = $1
       family = Socket::AF_INET6
+      bracketed = true
     end
     if prefix =~ /\A(.*)(%\w+)\z/
       prefix = $1
@@ -743,6 +744,12 @@ def initialize(addr = '::', family = Socket::AF_UNSPEC)
     end
     if prefixlen
       mask!(prefixlen)
+
+      if bracketed
+        warn "bracketed IPv6 address with a prefix length is an invalid address " \
+             "and will raise IPAddr::InvalidAddressError in a future version: #{addr}",
+             uplevel: 1
+      end
     else
       @mask_addr = (@family == Socket::AF_INET) ? IN4MASK : IN6MASK
     end
diff --git a/test/test_ipaddr.rb b/test/test_ipaddr.rb
index 3b73664..9a4e345 100644
--- a/test/test_ipaddr.rb
+++ b/test/test_ipaddr.rb
@@ -100,7 +100,9 @@ def test_s_new
     assert_equal("0:0:0:1::", IPAddr.new("0:0:0:1::").to_s)
     assert_equal("2001:200:300::", IPAddr.new("2001:200:300::/48").to_s)
 
-    assert_equal("2001:200:300::", IPAddr.new("[2001:200:300::]/48").to_s)
+    assert_warning(/will raise IPAddr::InvalidAddressError/) {
+      assert_equal("2001:200:300::", IPAddr.new("[2001:200:300::]/48").to_s)
+    }
     assert_equal("1:2:3:4:5:6:7:0", IPAddr.new("1:2:3:4:5:6:7::").to_s)
     assert_equal("0:2:3:4:5:6:7:8", IPAddr.new("::2:3:4:5:6:7:8").to_s)
 
@@ -772,4 +774,16 @@ def test_raises_invalid_address_error_with_error_message
     end
     assert_equal('invalid address: INVALID', e.message)
   end
+
+  def test_s_new_bracketed_with_prefix_is_deprecated
+    assert_warning(/will raise IPAddr::InvalidAddressError/) {
+      assert_equal("2001:db8::", IPAddr.new("[2001:db8::]/32").to_s)
+    }
+    assert_warning(/will raise IPAddr::InvalidAddressError/) {
+      assert_equal("2001:db8::", IPAddr.new("[2001:db8::]/ffff:ffff::").to_s)
+    }
+
+    assert_warning("") { assert_equal("2001:db8::", IPAddr.new("[2001:db8::]").to_s) }
+    assert_warning("") { assert_equal("2001:db8::", IPAddr.new("2001:db8::/32").to_s) }
+  end
 end