Eastmond broke into the St Helens side in 2008 and has gone on to make 42 appearances for the club since, scoring a total of 401 points in that time. He has represented England four times, earning his first cap in 2009.
A skilful player, Kyle has a strong kicking game, a natural line running ability and a keen eye for the gap in a defensive line.
"I am very excited and privileged to be offered the opportunity to not only play rugby union but to play at such a prestigious club with such a great history and forward ambition as Bath.
"This has been a long, hard and carefully thought out decision, with my family being an invaluable source of support, but I feel that the opportunity presented to me was too good to miss and I am thoroughly looking forward to the challenges ahead.
"Obviously I am still contracted to St Helens for the remainder of this season and am 100% committed to contributing to a successful year for the Club. I am aware that there has been a fair amount of press speculation over the last few weeks but I hope that this announcement will now allow me to focus all my energies on my performances for the Saints until the end of my contract."
Director of Rugby, Sir Ian McGeechan, said "Kyle is an exceptionally promising young talent, who has already developed a set of skills in League that will prove very useful in Union. He is a hard working player, who will fit in well with the squad here, and we are all very excited about the opportunity to work with him."
Head Coach, Steve Meehan, commented "We are looking forward to having Kyle join us here at Bath, and to developing him into a top class Union player. The experience he has gained from performing at the top level and under pressure will prove very useful. He is very keen to develop his game further and apply the good skills he already has to the Union game."