The move comes as the Premiership champions prepare for a pre-season Stateside visit.
‘Chelsea is showing a greater commitment to the US soccer market than any other top European club, so it is natural that we will seek relationships with top brands in specific areas to assist our business aims,' said Chelsea director of comms Simon Greenberg.
He added: ‘This will be our third consecutive year of pre-season training in the US, and we will be returning next year.
‘We want to capitalise on this from a brand and business perspective but we are also committed to promoting soccer generally in the US and working on grassroots programmes.'
H&K's Chicago V-P of sports Patrick Sandusky, who formerly worked in the agency's London office, heads the account, reporting to Greenberg.
‘Football is now far less middle-class in the US, especially with the Hispanic influence,' Sandusky said. He added: ‘We like winners over here. The fact that Chelsea are the two-time reigning champions will strike a chord.'
Chelsea follows a path previously trodden by Manchester United FC.
Chelsea's current chief executive Peter Kenyon was a key figure in Man United's US forays during his time at Old Trafford.