The 20-year-old centre, who chose rugby rather than following in his father's cricketing footsteps, is determined to succeed at the Arms Park. "I am totally committed to them," he said.
"They have opened my eyes to what professional rugby is all about."
Botham first got his chance with Cardiff after Peter Thomas, their millionaire benefactor, met his father as they followed the European team around at the Ryder Cup in Valderrama.
Botham played two friendlies which persuaded Terry Holmes, the club coach, to give him a permanent deal.
The Cardiff manager, Peter Manning, said: "He has a good attitude and is a very determined, competitive person who wants to make it as a professional rugby player."
Aberavon have signed the New Zealand-born No 8 Guy Phillips from Bridgend. Phillips, 22, son of the Bridgend coach, John Phillips, was released by his father as Bridgend had exceeded their quota of overseas players.Reuse content