The commune is situated in the eastern foothills of the Bodoc Mountains [ro], at an altitude of 588 m (1,929 ft), on the banks of the river Cașin.

It is crossed by national road DN11B [ro], which connects Târgu Secuiesc to Cozmeni, Harghita County.

In August 1940, under the auspices of Nazi Germany, which imposed the Second Vienna Award, Hungary retook the territory of Northern Transylvania (which included Sânzieni) from Romania.

Towards the end of World War II, however, the commune was taken back from Hungarian and German troops by Romanian and Soviet forces in September 1944.

In 1950, after Communist Romania was established, Sânzieni became part of the Târgu Secuiesc Raion of Stalin Region.