Rice, 20, was capped three times by Ireland, but all of his appearances were in friendlies, which allows him to swap his worldwide allegiance to the country of his birth.
"This afternoon I telephoned both (Ireland manager) Mick McCarthy and (England boss) Gareth Southgate to inform them of my decision to submit a written request to Federation Internationale de Football Association for the transfer of my global registration from the Republic of Ireland to England", Rice said in a long statement on his Twitter account.
Rice has made 23 Premier League appearances for West Ham this season, scoring one goal.
In the statement, he said that he has "equal love for both England and Ireland", which is why it taken him months to make the decision.
Rice qualified for Ireland through his grandparents from Cork.
"I have also spoken to Martin O'Neill, Mick McCarthy, Gareth Southgate and to friends and colleagues within the game whose opinion I respect". I am a proud Englishman, having been born and raised in London.
"I fully accept that some Irish supporters will be disappointed ..." Particularly not for a young lad who never dreamed of being in this position.
"Declan rang me today and said he has chose to give it a go with England".
"Ultimately, it is a personal decision I have made with my heart and my head, based on what I believe is best for my future. Good luck to him", he said.
Republic of Ireland manager McCarthy confirmed that Rice had been in contact over his decision. But the competition is greater for a starting place - especially after England reached the semifinals at last year's World Cup and will play in the Uefa Nations League final four in June.