Larijani, a Principlist, won the post on Sunday with 173 votes in a race against Mohammad Reza Aref who had been fielded by the Reformist camp and secured only 103 votes.
Independent members were apparently crucial in electing Larijani because no single party holds an overall majority in the new Majlis.
He was also the speaker of the previous parliament dominated by Principlists.
A vote for a permanent speaker is due to be held in the next few days, after the full house approves the credentials of individual MPs, as required by Iran's constitution.
On Saturday, President Hassan Rouhani praised Larijani for supporting Iran's nuclear pact and called for greater "interaction" between parliament and the government to "solve the problems and crises of the country."
The 290-member parliament was inaugurated Saturday in the presence of 265 members, with three seats vacant after votes for two MPs were nullified by Iran's constitutional oversight body, the Guardian Council, and a third member died in a car accident.
The new parliament includes a record 18 women, an achievement Rouhani said made him "very happy."
In a message to the opening, Leader of the Islamic Revolution Ayatollah Seyyed Ali Khamenei hailed the big turnout of the people in the February elections as the "re-allegiance with Islamic Republic."
The Leader called on the elected legislators to be vigilant in the face of regional and international challenges.
"The turbulent state of the region and the world and the international adventurism of oppressors and their vassals have confronted the Islamic Iran with conditions more complicated than before," said the message, read to a packed parliament chamber.
"It is the revolutionary and legal duty of you to make the parliament a stronghold against the schemes, charms and impudently excessive demands of the Arrogance," the Leader said.