The butler is able to bring guests items such as towels and toothbrushes, freeing human staff for other activities, the company said.
But the UK hotel workers' union said the butlers were "no replacement for top quality customer service".
"With a night at one of Starwood's luxury London hotels costing around £290, the global hotel chain would do better ditching the gimmicks and investing in better pay and training programmes for its worldwide workforce," said Hugh O'Shea, a spokesman for Unite.
"Starwood Hotels' 'butler robots' are a nifty little gimmick but can't ever replace the human touch expected by discerning hotel guests."
No tips Mr O'Shea called on the chain to invest instead in providing the living wage for its staff.
Starwood, which runs chains such as St Regis and Le Meridien, is using the sharply-dressed bots in its Aloft Hotel in Cupertino, California.
Named Botlr, the machines do not accept cash tips, but visitors can express their satisfaction by tweeting thanks using a specific hashtag.
But while the butlers, created by Silicon Valley-based start-up Savioke, can carry out tasks previously done by humans - they are not intended to replace entry level jobs, the hotel chain insisted.
"I see this as an enhancement to our customer service," Brian McGuinness, Starwood's senior vice-president for its Specialty Select brands, told the New York Times.
He added: "It's not going to be a replacement for our human talent."