Eastmond had long been linked with a cross-code switch and his current side have today confirmed that he will become Union's latest League recruit.
"He has decided to go to Bath - the players sat around and we talked about it," said St Helens coach Royce Simmons.
"He guaranteed us he was going to give us 100% for the rest of this season."
Eastmond has scored over 400 points in his 57 first-team appearances for St Helens since his debut in 2007.
The 21-year-old only has three England caps to his name but he was expected to become a mainstay of the international side over the coming seasons.
Instead he has opted to follow current England Rugby Union internationals Chris Ashton and Shontayne Hape in making the move to the 15-a-side version of the sport.
"Kyle has got everything going for him. There is no reason he can't be a great success," Hape told Sky Sports from England's Six Nations training base.
"I spoke to him when he came down to Bath, I said it wouldn't be easy and the most important thing is you have to start over again.
"You can't come across and think you are something special. You have to start from the bottom and work his way up.
"The old rugby heads will think 'who is this guy, who does he think he is?' You have to be humble, earn the respect of his team-mates and coaches. I think he understands that and has a good head on him.
"Everyone compares him with Jason Robinson because he is a similar type of player.
"Who knows, this time next year Kyle could be the one sitting here being interviewed in the England camp."
And with the next Lions tour still two-and-a-half years away, Eastmond could establish himself as one of the game's leading lights in time to make the trip to Australia in 2013.
While the likes of League stars Chev Walker, Karl Pryce and Henry Paul have failed to live up to expectation in recent years, Jason Robinson enjoyed a meteoric rise to the top after becoming a Test Lion in 2001 despite having only switched codes less than a year earlier.