U.S. State Department spokeswoman Marie Harf issued the call ahead of the second anniversary on Sunday of Byalyatski’s arrest.
She praised Byalyatski’s work to support victims of political repression and for cataloging human rights violations in Belarus.
Earlier, the UN Special Rapporteur Miklos Haraszti urged Minsk to "immediately and unconditionally" release Byalyatski.
Haraszti said his continued detention is "a symbol of the repression against human rights defenders."
Byalyatski helped found Belarus’ opposition Popular Front and heads the Vyasna (Spring) human rights group.
He was sentenced in November 2011 to four-and-a-half-years on tax evasion charges that his supporters say were politically motivated.
The charges stemmed from Byalyatski's alleged use of personal accounts in Lithuania and Poland to receive funding from international donors for human rights activism in Belarus.
Amnesty International has declared him a prisoner of conscience.