Australian clubs as well as NRL sides have been clamouring to sign the 20-year-old winger, who was one of the breakout stars of this year's World Series Sevens.
From Kempsey on the NSW north coast, Lancaster scored 20 tries in 34 matches to also catch the eye of Junior Wallabies selectors.
He was named in the Australian U20s squad who will play two matches against New Zealand in Wellington next week before their world titles starting next month in South Africa.
Lancaster had been working with the NSW Waratahs Academy when he was surprised by the opportunity to join the Sevens program, making his debut last year.
The Australian men qualified last weekend for the Sevens competition at next year's Olympics in Paris, which Lancaster could still be a part of following the Super season.
The Rebels said they would be "encouraging" him to play in the Olympics, with their coach Kevin Foote a former South Africa Sevens representative.
Lancaster follows former Sevens representatives Lachie Anderson and Joe Pincus to Melbourne, with both making an impact in Super rugby this season.
"The Rebels play an exciting and expansive style of rugby which will suit me as an outside back," Lancaster said in a statement.
"Obviously seeing the success of Joe Pincus and Lachie Anderson as former Sevens players, I can take great confidence in the Rebels ability to help me transition successfully back into XV's.
"Both players are role models of mine and I hope to follow in their footsteps.
"I have loved watching them play this year and look forward to hopefully playing alongside them next year on AAMI Park.
"I have high hopes for next year to be successful alongside a handful of new signings complimenting a strong core group of players there."
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