Elon Musk won a major victory in Delaware court on Wednesday, when a judge ruled the Tesla Inc. chief executive didn’t act unlawfully in the electric-vehicle maker’s roughly $2.1 billion takeover of SolarCity Corp.

A group of Tesla shareholders had alleged that Mr. Musk controlled the 2016 acquisition while having a financial interest in both companies, that many of its directors were conflicted, and that Tesla overpaid for the home-solar company. Mr. Musk has said he didn’t dictate the deal process or price and recused himself from the shareholder vote.