BT, Britain's dominant fixed-line telecoms group, sued Google in 2011 over six patents related to mobile technology.
But in a lawsuit filed on Wednesday in a California federal court, Google asserted four patents of its own.
Google spokeswoman Niki Fenwick said the company also filed a separate lawsuit against BT in a U.K. court.
"We have always seen litigation as a last resort, and we work hard to avoid lawsuits," Fenwick said in an email. "But BT has brought several meritless patent claims against Google and our customers - and they've also been arming patent trolls."
A BT representative said the company does not comment on pending litigation.
BT has accused Google of infringing BT's mobile patents through numerous products, including the Android mobile platform, Gmail email service and Google Maps. That lawsuit is still pending in a Delaware federal court.
In addition, Google was sued in a Virginia federal court by Suffolk Technologies LLC and one of the patents being asserted by Suffolk was previously owned by BT, according to court filings. A Suffolk attorney declined to comment.
In Google's U.S. lawsuit, the company said BT violates Google patents that cover technology such as networking and making phone calls over the Internet. Google obtained at least one of the patents in its lawsuit from International Business Machines Corp in 2010, according to a government database.
IBM representatives could not immediately be reached.
The case in U.S. District Court, Central District of California is Google Inc v. BT Americas Inc, BT Conferencing Inc, BT Ins Inc and Ipanema Technologies Corporation.