James Botham - the son of former Cardiff rugby player Liam - is in the Wales Under-18 team to play Scotland in Swansea on Friday.
Sir Ian, 60, does not find it strange that his grandson will be wearing red.
"He was born in Cardiff. He always wanted to play for Wales for as long as I can recall," said the ex-all-rounder.
"I'm really looking forward to it. I think it will be wonderful, a great occasion for James and one that I think he's very excited about, as are the family. I've got a red shirt with me."
Botham, who took 383 wickets and scored 5,200 runs in Tests for England, will watch James make his debut on at St Helen's - where he played county cricket against Glamorgan for Somerset.
James Botham is one of four players making their debuts for Wales, along with Harri Dobbs, Ben Thomas and Dane Blacker.
The match is part of a series which culminates in a game against England Under-18s on 6 April.
Sir Ian said he had not been tempted to persuade James to switch allegiance.
"Never, it's nothing to do with me," he added.
"I'm grandad, and grandad is going to come down and watch him and support him every game. Whether he's playing or on the bench it doesn't matter - all the family that can get down here will be here.
"And when Wales play England my allegiances would be with my grandson."
England cricket legend Sir Ian Botham is looking forward to supporting his grandson James when he makes his rugby debut for Wales.