About frames moving at constant velocity with respect to inertial ones

It is well-known that in Newtonian mechanics, a system moving with
constant velocity with respect to an inertial one can be proved also
inertial. Have we a similar assertion valid in Einstein's 1905
Relativity (1905R)? According to 1905R, a system moving with constant
velocity with respect to an inertial one can be proved also inertial?
If the answer is positive, I want to know who, when and where proved
it by first time. If those data are not available, I want then any
valid 1905 Relativity deduction of it.

.

Relevant Pages