Tesla Motors Inc.’s second-quarter sales jumped 52%, higher than the company had forecast, giving the Silicon Valley auto maker important momentum ahead of the debut of its Model X sport-utility later this year.

Tesla’s shares rose 2% to $274.75 in morning trading following the disclosure Thursday. Tesla sold 11,507 Model S sedans for the three months that ended June 30, a company record. The global sales results were better than the 10,000 to 11,000 forecast the company had given two months ago in its first quarter earnings...