A product of the White Hart Lane academy system, the 19-year-old midfielder has already spent time in America during pre-season training.
He caught the eye during that spell, netting twice, and the Fire have moved quickly to secure his services longer term.
No details regarding the length of the loan agreement have been released, but Ward will return to the USA after working his way back from an operation to repair a fractured metatarsal.
Fire head coach Frank Yallop said in a statement: "Grant is a young, technically-gifted player.
"I watched him play in England, and throughout our pre-season training camp Grant consistently proved that he has what it takes to do well in the MLS.
"We look forward to him joining our squad once he has recovered from his injury."
Fire technical director Brian Bliss added: "Grant's injury is an unfortunate one but we fully expect him to bounce back.
"We saw the type of player Grant is while he was with us during pre-season, and we're here to support and do what we can to help him get back to full fitness so he can join us later this season."