An attorney for Donald Trump’s son-in-law Jared Kushner on Friday said certain documents requested by a Senate committee investigating the Trump campaign’s ties to Russia weren’t relevant to its initial request, disputing the panel’s suggestion that his client hadn’t wholly fulfilled its demand for the material.

In a letter Thursday to Abbe Lowell, Mr. Kushner’s lawyer, Senate Judiciary Committee Chairman Chuck Grassley (R., Iowa) and Sen. Dianne Feinstein (D., Calif.) wrote that Mr. Kushner hadn’t turned over all the documents...