Penrith Panthers have become just the second club to win more than one under-20s grand final with a 34-18 win over Manly.

The Panthers raced in six tries to three to win their second title in three years. In what was the first time two Sydney sides met in the premiership decider, Penrith finished the season with just four losses on their way to the minor premiership. It was the fourth time Manly have lost to Penrith this year under first-time Panthers coach Cameron Ciraldo.

"Really pleased for the boys, a lot of hard work has gone into this," Ciraldo said. "It was tough [on Sunday]. We had plenty of adversity thrown our way. If you look at the blokes on the sidelines about eight or nine of them were pencilled into our starting team at the start of the year. It was a real tough effort. It wasn't our best performance."

Penrith finished the game without strike centre Robert Jennings who left the field with an ankle injury 10 minutes into the second half in hot conditions.

About a dozen Panthers players are eligible for the under-20s again next year.

"All those kids who played have a big opportunity ahead of them," Ciraldo said. "They are all here next year. If they keep working as hard as they are now, I am sure there will be a number of them playing first grade in the next couple of years."

The Panthers had limited chances in the first half but scored with almost every set inside Manly's 20-metre zone.

Hooker Soni Luke scored the opening try after just three minutes when a simple dummy split the Manly defence for him to score.

Penrith extended their lead 10 minutes later when five-eighth Tyrone May dummied his way to the try line to extend the lead to 12.

The only two players with NRL experience in Tom Trbojevic and Jennings would score their respective next points for their sides. For fullback Trbojevic it was his 21st try in 14 games.

Manly trailed by four points at half-time when back-rower Luke Garner scored just before the break.

Penrith stamped their class in the second half racing in two tries in eight minutes to secure the victory. Maliko Filino and Jarome Luai each crossed to give Penrith a 28-12 lead.

Manly's Liam Knight scored a late try for the Sea Eagles before Fuimanu Enese scored the final try of the game for the Panthers.

Luke picked up the Jack Gibson Medal as man of the match.

"It was a really good experience to play on grand final day," Luke said. "A bit of relief when the siren went. It was so hot out there. At times it was ridiculous but we got through it."