The 20-year-old has been training with the Sky Bet Championship club for the last week and has scored two goals in two 45-minute pre-season friendly appearances.
Ward, who has yet to feature for Spurs’ first team, impressed when on loan at Sky Bet League 1 Coventry last season and becomes the Millers’ eighth summer signing.
Boss Steve Evans said: “Everyone inside the football club, none more so than the players, will be delighted to hear the news that we have now completed the signing of Grant Ward from Tottenham Hotspur.
“First and foremost I must thank Mauricio Pochettino and Paul Mitchell at Spurs for approving the loan.
“There has been significant competition for the lad’s signature, but we took the opportunity to have him with us for a few days, and I was delighted when he informed me this morning (Monday) that he has chosen us versus several Championship options.
“Grant has a big career in front of him and he now has the opportunity to make steps up the ladder with a successful loan spell with the Millers.”