Syria's membership in the Arab league has been suspended since 2011, when the Assad regime launched a harsh crackdown on pro-democracy protests, sparking nine years of civil war.
"On the occasion of the Syrian Arab Republic's National Day, it is my pleasure to express my congratulations, wishing further progress and prosperity to the fraternal Syrian people," Ghazouani said in his letter.
The Syrian Embassy in Mauritania holds regular meetings with Mauritanian officials.
*Ahmed Asmar contributed to this report from Ankara