President Donald Trump’s announcement of his first pick for the Supreme Court in an unusual prime-time TV event on Tuesday drew 33 million viewers across broadcast and cable television, according to Nielsen.

The White House said millions more watched online as he nominated Judge Neil Gorsuch to succeed the late Antonin Scalia.

The timing of Mr. Trump’s announcement wasn’t unprecedented. President George W. Bush unveiled his nomination of Chief Justice John Roberts in prime-time in 2005.

Mr. Trump, a former reality TV star, orchestrated a high-drama event and appeared to relish keeping the press guessing. The two finalists raced to D.C. with CNN and other news outlets hot on their trail to find out who would be chosen to wear the black robe on the high court.

In terms of ratings, the broadcast networks have nothing to complain about. CBS led the way with 8 million viewers watching the announcement, making it the most-viewed program of the day for the network. On NBC, the announcement averaged 5.9 million viewers and ABC drew 5.1 million viewers. On cable, Fox News Channel came in first with 7.6 million viewers, easily beating CNN’s 2 million viewers and MSNBC’s 1.6 million.

President Trump’s penchant for splashy events and creating suspense has media industry observers anticipating he may seek a prime time platform more than previous presidents.

Broadcasters typically are reluctant to preempt their evening lineups for presidential announcements or press conferences because it means lost ad revenue and creates scheduling headaches. CBS had to rearrange its Tuesday schedule and moved a special out of the 8 p.m. hour to 10 p.m., replacing it with a rerun. Fox had to pull a new episode of its sitcom “New Girl.”

But Mr. Trump could also be a ratings draw as a lead-in to other programming.

“It is a prime time challenge. The fact is this guy’s ratings are really off the roof,” said Frank Sesno, director of George Washington University’s School of Media and Public Affairs and a former CNN correspondent.

Asked at a press briefing if President Trump would continue to seek out prime time for announcements, White House Press Secretary Sean Spicer, “on the airtime issue, that’s something we’re not particularly suffering from when it comes to attention.”

The numbers for Tuesday’s event topped the 30.6 million viewers who watched President Trump’s inauguration, although that event ran much longer and wasn’t in prime-time.

Mr. Spicer said 11 million people also watched Judge Gorsuch’s nomination on Facebook Live. Facebook could not confirm the figure. A spokeswoman said the social network hasn’t done its own tally of views.

For now, the broadcast networks say they will take a wait-and-see approach on giving up valuable prime time real estate to the president.

“If there are subsequent events, we’re going to evaluate them as we would anything else – based on the news,” one network news president said.

—Michael C. Bender contributed to this article.

Write to Joe Flint at joe.flint@wsj.com