Casey was on the bench as Graham Potter’s Hammers wrapped up their Premier League Summer Series games with a 2-0 win over Bournemouth in Atlanta, but he is now part of the Swans set-up.
An England Under-20 international, Casey has made two substitute appearances in the Premier League for West Ham but has been allowed to leave in order to gain first-team experience.
Swansea are short of options at the back for the start of their campaign so the move comes as a boost to boss Sheehan.
Richard Montague, the Swansea director of football, added: “Kaelan will provide valuable depth for us in an area we had previously identified as needed additional resource this summer.
“He has already had a taste of Premier League football, which underlines how he is viewed by West Ham, and in our discussions with him his appetite to challenge himself has been clear.”
Club captain Ben Cabango is doubtful for the Boro trip having been troubled by a calf injury during pre-season.
Fellow centre-back Ricardo Santos is also sidelined by injury, as is youngster Filip Lissah.
Cameron Burgess, another of Swansea’s summer recruits, was Sheehan’s only fit senior centre-back, with under-21 player Arthur Parker and midfielder Jay Fulton used at the heart of defence at times during pre-season.
Swansea are still looking for a number of new signings following a raft of summer departures, with a striker another top priority following the exits of Jerry Yates and Florian Bianchini last week.
