The show, which made veiled references to the absence of Ant McPartlin after he was charged with drink-driving, peaked with 8.6 million viewers.
The episode was the second most viewed of the series, bested by the 7.8 million that watched the show's 100th episode on March 3, but bagged the biggest audience share of the series, the broadcaster said.
The show's highest audience share before Saturday night was on February 24, when it pulled in 37%.
Overnight figures include those watching on ITV +1.
Donnelly was without his normal sidekick after McPartlin stepped down from his TV commitments when he was charged by police last month.
Presenting on his own for the first time in the programme's history, Donnelly oversaw an almost seamless live 90-minute show with his only mishap dropping a carton of popcorn.
At one point he said following the news had been "unavoidable" in recent weeks, but did not explicitly mention why McPartlin was missing.
He also joked he had "twice the amount of work to do" and later added: "I'll have to do it myself, like everything else this week," in a quip related to improving guest musician Paloma Faith's performance.
Later when presenting a segment from an empty ITV studio next to Saturday Night Takeaway's, Dec joked not to worry - "It's not going that badly."
McPartlin was not completely absent from the programme. He appeared in the opening titles and also in the continuation of pre-recorded sketch Saturday Knight Takeaway which also starred Joanna Lumley in her regular role.
Although officially hosting solo, Donnelly was supported by presenter Davina McCall as part of a new segment related to McCall's own show, This Time Next Year.
Regular support presenters Stephen Mulhern and Scarlett Moffatt also lent a helping hand as did guest announcer Stephen Merchant, who referenced McPartlin's absence on several occasions.
He asked Donnelly if he needed a hand presenting as there was "a little bit of space on the stage", and joked at one point: "I don't care what the producers are saying back here, I think you're doing a great job."
Following the airing of the pre-recorded sketch which saw the duo reunited, Merchant said: "That other guy was good, you should do more stuff with him."
Dozens of fans - including fellow broadcasters Piers Morgan, Susanna Reid and Dermot O'Leary - congratulated Donnelly on his successful performance on social media.
O'Leary wrote on Twitter that he had "nailed it" - a call echoed by a number of other broadcasters including Rylan Clark and Gok Wan.
Reid wrote: "He's really easing into it and getting more comfortable as he realises how much the audience is with him."
Hundreds of viewers agreed saying he had done his friend "proud".
@Mitchinator9339 wrote on Twitter: "Dec did such a good job on #SaturdayNightTakeaway tonight.
"I know Ant would be proud of you. Here's hoping that he will do as great a job at Florida next week."
@SamdySonxxxxx posted: "@antanddec @itvtakeaway Dec you did yourself & Ant proud, it was a fabulous but emotional show, keep smiling. looking forward to next week."
@LauraJH55 tweeted: "So fantastic watching #SaturdayNightTakeaway tonight, Dec @antanddec did amazing with a brilliant end of the show show."
After the show, Donnelly said he was grateful to his fans for their messages of support.
He wrote on Twitter: "Thanks for all your lovely messages tonight, they are VERY much appreciated, I'm glad you enjoyed the show."
The 42-year-old, who recently confirmed he is going to be a father, will also present next week's final episode in Florida.
McPartlin will appear in court on April 4.