Hosted by David Muir, the hour-long special celebrated the late Williams, who died Monday of an apparent suicide.
The special, which aired at 10 p.m., earned a rating of 1.7 in the key 18-to-49-year-old demographic, making it second-most watched in the desired category.
According to data provided by Canvs, a social TV platform created by social media insights firm Mashwork, there were 5,546 tweets about the special. About 41% of the tweets used emotional language, with many people sharing feelings of both happiness and sadness.
"This 20/20 episode on Robin Williams is tugging on my heart strings big time," one user wrote on Twitter.
ABC was the third-most-watched network of the night with roughly 5 million viewers. With a rating of 1.3 in the key demo, the network tied for second-most watched with Univision in that category.
NBC's "America's Got Talent" earned the No. 1 spot of the night in the key demo, posting a solid rating of 2.0.
About 9.5 million people tuned in to another round of Season 9's quarterfinals, elevating the show to the most-watched program overall. There were 37,422 tweets about the episode, according to Canvs.
The competition show, which showcased 12 different acts, began with a short tribute to Williams from judge Howie Mandel.
"Tonight, as we watch every young person come out on this stage and go after their dreams ... hopefully they will light a small light to help fill that void, that huge void, that Mr. Robin Williams is leaving behind," Mandel said.
The network, which won the night with 7.8 million viewers, also aired an episode of "Food Fighters."
Adam Richman's culinary competition show earned a rating of 1.1 in the key demo and roughly 4.4 million people tuned in.
CBS' "NCIS" was the second-most-watched program of the night, with roughly 8 million viewers. The show earned a rating of 1.0 in the key demo. The network was second-most watched of the night, averaging 6.3 million viewers.
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