"Tesla Semi truck unveil set for September," Musk said on Twitter. "Team has done an amazing job. Seriously next level." He did not provide any details, including when the truck would be available for purchase.
The truck would add to the ambitious production schedule underway at Palo Alto, Calif.-based Tesla, which also is gearing up to build its widely anticipated Model 3 mid-market sedan.
Musk previously had indicated that the semi truck was coming and that it likely would have some autonomous-driving features now included in Tesla's passenger cars.
Musk also tweeted Thursday that Tesla expects to unveil a pickup truck in 18 to 24 months.
In outlining the company's "master plan" last July, Musk said heavy-duty trucks were among the other types of electric vehicles needed in the marketplace and that Tesla expected to unveil its truck this year.
"We believe the Tesla Semi will deliver a substantial reduction in the cost of cargo transport, while increasing safety and making it really fun to operate," he said then.
Others also are working on bringing a zero-emissions cargo truck to market, such as Mercedes-Benz and its Urban eTruck, unveiled last year but not expected to reach full production until 2020, and Nikola Motor Co.'s hydrogen fuel cell truck, which the Salt Lake City company showcased in December.
The Otto division of Uber Technologies Inc. has been developing heavy-duty semi trucks with self-driving capabilities.
Tesla, which already builds its mainstay Model S sedan and Model X sport utility vehicle, briefly became the largest U.S. automaker by stock market value early this week when its market valuation surpassed that of General Motors Corp.
Tesla's stock closed at $304 a share, up $7.16 or 2.4%, Thursday, giving the firm a market value of $49.6 billion.
2:30 p.m.: This article was updated with information about other companies that are working on zero-emissions trucks. Another Musk tweet about the Tesla pickup truck was added.