Three rafts flipped on the Naranjo river Saturday around 3 p.m. and the five victims were carried away downstream, according to the Judicial Investigation Department.
Other passengers managed to cling to the rafts and some were rescued by another guide in a kayak.
The Judicial Investigation Department identified those who died as Ernesto Sierra, Jorge Caso, Sergio Lorenzo and Andres Dennis.
The four were celebrating the bachelor party of their high school friend.
The local guide was Kevin Thompson Reid.
In total there were 14 tourists aboard the rafts and five guides.
Authorities said the river was swollen by rains and the National Emergency Commission maintained an alert in the area due to the possibility of flooding.
The Americans had arrived in Costa Rica Oct. 18 and had been renting a house in Playa Hermosa de Jaco, according to the government.
The Red Cross said via Facebook that the rafts overturned near Liverpool de Quepos.
Costa Rica President Carlos Alvarado expressed his dismay about the accident on Twitter.
Expreso mi profunda consternación por el accidente ocurrido esta tarde en el río Naranjo en Quepos. pic.twitter.com/r9HMW6SpjY— Carlos Alvarado Q. (@CarlosAlvQ) October 21, 2018