WASHINGTON—The White House will hold a social media summit on July 11, aiming to have a “robust conversation” about online platforms at a time when they are under increasing attack in Washington.

White House confirmation of the summit came after President Trump dialed up his criticism of tech companies on Wednesday morning. He said in an interview on Fox Business Network that online giants such as Google and Facebook Inc. are “trying to rig the election,” while Twitter Inc. is making it harder for users to follow him.

The...