The bird was rescued from Hove beach
A passer-by said the bird stood "with its head under its wing" before being placed in a box by a concerned member of the public until the volunteers arrived.
Volunteers say the bird, which is most commonly found in the Caribbean and on the west coast of Africa, was underweight at just 760g.
Lourdes Cortes, veterinary surgeon for East Sussex WRAS said: "We are being very guarded about the birds' chances of survival due to the degree of weight loss.
"The bird has a good attitude and being a good patient, so we are trying our best to build the bird up. It's very much one step at a time."
The brown bobby is the most common and widespread of the Booby species, but very unusual in the UK.
The last one to be seen on British soil was found in Cornwall in 2019.
East Sussex WRAS say this is the second bobby it has had in its care after rescuing a red footed bobby in 2016.
Trevor Weeks MBE, founder and operations director of East Sussex WRAS said, "Having one Booby to deal with was a surprise to get a second turn up here in Sussex is really unusual as these birds are found around from the Mediterranean and down onto the African coast as well as around the Gulf of Mexico over a pantropical range.
"We don't know where the bird came from originally nor what route it would have taken but we are aware that these birds have landed on shipping and traveled outside of their natural home range before as well as being blown off course in storms."
Once the bird is healthy enough, it is likely to be transferred to another specialist rehabilitation centre before potentially being flown back to a more natural location for its release.
Species range
R.C.