The 21-year-old Australian conservationist and his professional partner Witney Carson lifted the Mirrorball Trophy on Tuesday night after battling it out against Alix Earle, Jordan Chiles, Elaine Hendrix and Dylan Efron.
Speaking after the result was announced, Irwin cried as he said: "My sister said it best: thank you for changing my life!"
Carson, 32, added: "I'm so grateful for Robert, honestly - I already feel like I won with him as my friend and I can't thank you enough."
The ABC show's season 34 saw the couple perform a Quickstep to Jet's Are You Gonna Be My Girl for the Judges' Choice round, as well as a freestyle to Black and Gold by Sam Sparro and The Nights by Avicii.
In addition, the Instant Dance round saw them perform a cha-cha to DNCE's Cake by the Ocean.
They also performed a cha-cha to Cake by the Ocean during the Instant Dance round.
The finale saw all the previously-eliminated couples from the season return to dance to Tyler, The Creator's Sugar on My Tongue, Lady Gaga's Applause, and I Don't Dance by Alexander Jean, while season 33 champions Joey Graziadei and Jenna Johnson reunited for a routine soundtracked by Fabio Hager Sexteto's Tanguera.
Co-host Alfonso Ribeiro revealed the finale had broken several records for the show.
More votes were cast for the top two, Robert and Alix, than any top two in the show's history and the difference between them was closer than ever.
More than 72 million votes were cast during the finale, and more than 500 million in total throughout the series.
A few weeks ago, Irwin claimed he "almost died" during the team dance after accidentally inhaling a piece of confetti.
He told Entertainment Tonight: "I almost died tonight.
"I choked on a piece of confetti after the team dance. I went over to Witney, and I think they cut away, and I inhaled a piece of confetti. I couldn't breathe.
"I went over to (Carson and showed I couldn't breathe), and then I just went, 'Ugh!' And it just popped out! I'm not even kidding!"
And Irwin feels grateful to have had Carson as his dance teacher.
He gushed: "The amount of trust I feel for Witney is insane. I feel like this little team we've got together has never been stronger, and I'm just so grateful that (Witney) trusts me to take on a new challenge every week.
"It's a dream come true to find new sides to myself."
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