Standing at 6ft 6ins, the 20-year-old becomes the Millers' 11th signing of the summer and will spend the 2026/27 campaign competing with Ted Cann for the number one jersey as Rotherham target an immediate return from League Two.
Eyestone is a product of the FC Dallas academy and spent time at Duke University before turning professional with Brentford in 2024. He has since become a fixture for the Bees' Under-21 side, helping them reach the final of the Premier League Cup, and was promoted into the senior squad picture last summer.
His development has also caught the eye of the United States setup. Eyestone has represented the USA at Under-19, Under-20 and Under-21 level, and was called into the senior squad as a training goalkeeper during the country's 2026 FIFA World Cup campaign, giving him first-hand experience of a major tournament environment before he has made a competitive first-team appearance anywhere.
Brentford rewarded that progress last year with a new long-term contract, understood to run through to 2031, underlining how highly the west London club rate his long-term potential even as they look to fast-track his development elsewhere.
Rotherham had trialled two other goalkeepers over the summer, Newcastle United academy graduate Max Thompson and free agent Jake Turner, before turning to Eyestone as their preferred option to challenge Cann, who has come under scrutiny from supporters following a run of shaky displays last season.
The loan represents Eyestone's first taste of first-team football in England since joining Brentford, and gives him a clear pathway to regular minutes in League Two rather than continuing to develop primarily in Under-21 football at the Gtech Community Stadium.
Rotherham begin their league campaign away at Cheltenham Town on Saturday, with Eyestone in contention to be involved in the matchday squad for the opening fixture as the club look to bounce back from relegation at the first attempt.
The loan forms part of a wider summer rebuild at New York Stadium under head of football Steve McClaren, who has overseen an extensive overhaul of the playing squad following last season's drop out of League One.