The 23-year-old joined the Redcliffe-based club on a one-year deal in 2026 after previously playing 83 NRL matches for Brisbane.
Cobbo came to prominence when he scored 35 tries in two seasons for the Broncos in 2022 and 2023.
On his day, the Cherbourg Hornets product is a match winner - a trait that has led to him playing six State of Origin matches for Queensland and one Test for Australia.
This season he has averaged a career high 174m in six matches, while scoring two tries, setting up two and making five line breaks.
His statistics at the Dolphins measure up to his best ever but there have also been uneven performances, such as in last round's 23-22 loss to Penrith when he made errors in a disappointing first half before shining in the second.
Dolphins coach Kristian Woolf became a fan of Cobbo's soon after his arrival.
"Selwyn impressed me with the way he presented himself as a young man in the first conversation we had and with what he wants to do with his footy as well," Woolf told AAP.
"He impressed me with the way he turned up fit, the way he trained and how he fitted into the group really seamlessly. The players respect him and love him."
Cobbo's one-year deal keeps him in the frame for a possible move to an expansion club such as PNG Chiefs, who enter the NRL in 2028.
The Dolphins, however, do not regard Cobbo as a short-term player at the club.
"We believe this contract extension is just the start of what will be a long career for Selwyn in the Dolphins' jersey," said Dolphins CEO Terry Reader.
"Selwyn has already made his presence felt at the Dolphins - on and off the field.
"What has been most impressive is how well he has melded into our playing group and the club in general. He has been a great fit for the Dolphins."
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