Bramwell, who was part of the Wales Sevens squad which was crowned World Champions in 2009, has previous experience with Ebbw Vale and Pontypridd with the 27-year-old providing Keys with plenty of back line experience.
"Aaron is an excellent signing for us," said Keys head coach Greg Woods.
"He is a very good player, will add pace to our back line and will really strengthen our squad."
Bramwell, who can also play centre, helped guide Neath to last season's SWALEC Cup final where they were beaten by Pontypridd.
He will now look to push for a place in Woods' squad as Keys prepare for their campaign with four pre-season fixtures.
They begin with a pair of matches against Gwent rivals Blackwood before travelling to Merthyr and then hosting English National League One side Coventry on August 17.
The competitive action then kicks off with a home British & Irish Cup play-off against Newport on the last day of August.
The winners of that match plays Bedwas at Bridge Field for a place in the competition proper.
Bramwell aside, Rhys Jones has also joined from the Scarlets and will compete with his brother Dorian for the fly half shirt while back rower Reuben Tucker has signed from Cardiff.