Assad won re-election in June amid criticism by Western nations that the election was "a sham."
At Wednesday's swearing-in ceremony, which was broadcast on state television, Assad placed his hand on the Koran and promised to "respect the country's constitution, its laws and republican system and to safeguard the interests of the people and their freedoms."
Assad denounced the Arab Spring in his inauguration speech, declaring, "They failed to convince you that they care about the interests of the people and their rights, they failed to make you feel that you need guardians over you to manage your affairs and those of your country."
The Syrian uprising in March 2011 was met by government-directed violence, which then led to a protracted and deadly ongoing civil war involving rebels, government forces, and extremist groups.
British Foreign Office Minister Mark Simmonds argued that Assad's re-election campaign in the midst of a civil war was intended "to sustain his dictatorship."