A one-time regional governor in Ukraine, Saakashvili dramatically fell out with Zelensky's predecessor Petro Poroshenko over allegations that Kiev was failing to fight endemic corruption.
The bitter dispute saw Saakashvili stripped of his Ukrainian passport in July 2017 with Kiev accusing him of trying to stage a coup sponsored by allies of ousted Kremlin-backed leader Viktor Yanukovych.
In February 2018, the former Georgian president was detained at a Kiev restaurant and flown to Poland.
Several months earlier he had crossed the Polish-Ukrainian border despite many border guards deployed to stop him from doing so.
Zelensky restored Saakashvili's citizenship in a decree posted on the presidential website.
"Thank you, President Zelensky! Glory to Ukraine!" Saakashvili wrote on Facebook.
In 2018, Saakashvili, who lives in the Netherlands, urged European leaders to introduce sanctions on Poroshenko, whom he accused of violating his human rights.
Saakashvili was president of tiny ex-Soviet Georgia from 2004 to 2013. He is also wanted by Tbilisi on charges of abuse of power that he strenuously denies.