Sharaa was also empowered to form a temporary legislative council for a transitional period and the Syrian constitution was suspended, according to an announcement made by the military command which led the offensive against Assad.
The decisions emerged from a meeting of military commanders who took part in the assault, a campaign spearheaded by Sharaa's Islamist Hayat Tahrir al-Sham (HTS) group - a former al Qaeda affiliate.
Addressing the conference, Sharaa said the first priority in Syria was to fill a vacuum in government "in a legitimate and legal way".
Syrians celebrated in the streets after al-Sharaa was named president. (EPA PHOTO)
He also said civil peace must be preserved through transitional justice and preventing displays of revenge, that state institutions - foremost among them military and security forces - be rebuilt, and that economic infrastructure be developed.
Sharaa has pledged to embark on a political transition including a national conference, an inclusive government, and eventual elections, which he has said could take up to four years to hold.
Wednesday's announcement did not say when the new legislative body might be picked, or provide any new details for a timeline for the transition.
Fawaz Gerges, Professor of International Relations at the London School of Economics, said the declaration had "formalised his status as the strongman ruler".
"My take is that HTS and Sharaa intend to consolidate single-party Islamist rule."
HTS emerged from the Nusra Front, Al-Qaeda's affiliate in the Syrian civil war, until it cut ties in 2016
The declaration announced that "Sharaa has assumed the presidency of the country in the transitional phase" and would "carry out the duties of the presidency of the Syrian Arab Republic, and represent it in international forums".
The new legislative council would carry out its tasks until a new constitution is adopted. The parliament elected under Assad last year was formally dissolved.
The declaration also reiterated previous steps dissolving Assad's Baath Party and his state security apparatus, and said that rebel groups which fought him during 13 years of war were to be dissolved and merged into the state.
The announcements came at a meeting declared "The Conference for Announcing the Victory of the Syrian Revolution". It was attended by ministers from the interim government appointed by HTS in December, and was not publicly announced ahead of time.
