After weeks of speculation, NSW State of Origin No.7 Southwell confirmed on Monday that she would not take up the option for the final two years of her Newcastle deal.
Instead, the 20-year-old will move north and partner retiring great Ali Brigginshaw in the Broncos' halves next year, before taking full charge of Brisbane's attack in 2027.
Southwell's exit comes as a significant blow to the Knights, who lost Tamika Upton to Brisbane last summer and finished a distant third.
Above them, both Brisbane and the Sydney Roosters only lost games to each other, with the Broncos winning a thrilling decider 22-18.
"They're obviously a really professional club and they've been around the women's space for a long time," Southwell said on Monday.
"That really enticed me. And the style of footy and the way they have been playing the past few years has been something cool to watch.
"But the biggest thing for me was I could see the girls were learning here. I can't wait to learn, learn from the girls, learn from the coaches."
Brisbane have lost five-eighth Gayle Broughton and Karyn Murphy Medallist Mele Hufanga to the Warriors next year, as well as lock Keilee Joseph to Parramatta.
But Southwell's signing ensures they will stay strong.
In a lopsided 2025 regular season, no other side got within 10 points of the Roosters or Brisbane in a game.
And while the Roosters were pushed by Cronulla in the finals, the Broncos ended the regular season with four times as many points as the last-placed Wests Tigers.
NRL chief executive Andrew Abdo insisted ahead of the finals it was too early to say whether the NRLW has become too lopsided, in its first season of going from 10 teams to 12.
But the likes of Andrew Johns even called for the NRL to block Southwell's move to Brisbane, fearing it would only add to the issue.
"If Jesse goes to Brisbane you may as well just have one round, just have a grand final: Brisbane against the Roosters," Johns said on a Nine podcast in August.
"Take away my loyalty to Newcastle ... for the good of the competition this cannot happen, because they will be absolutely unbeatable.
"Her kicking game is miles ahead of anyone else's in the game ... put that into Brisbane with their strong forward, Tamika at the back, Julia Robinson.
"No one is going to get near them. Every week they will be winning by 40 or 50 points, which will be sad."
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