Miqati, a former public works minister, was the opposition's nominee for the post.
The announcement by President Emile Lahud's spokesman Rafiq Shalala on Friday came two days after Prime Minister-designate Umar Karami gave up on efforts to form a cabinet after nearly seven weeks.
Miqati won the position after Lahud polled legislators or their representatives in the 128-member parliament.
Shalala said the president consulted the speaker of parliament about the results and then summoned Miqati to the presidential palace and asked him to form the next government.
Miqati gained the largest number of votes, saying he had won the support of 58 lawmakers.
Miqati gained the largest number
of votes in parliament
The number of those who backed his opponent, staunchly pro-Syrian Defence Minister Abd al-Rahim Murad, was not disclosed.
The government's main task now is to steer an electoral bill through parliament and call a crucial election.
Opposition leaders have said they are confident the vote will end pro-Syrian dominance of the legislature.
"We will be the symbol of moderation and national unity," Miqati declared from the presidential palace after being appointed by the president.
"I say it from here that the hand is extended and the heart is open so that we all cooperate in the Lebanese interest," he said.
Lebanon, in turmoil since the 14 February killing of former prime minister Rafiq al-Hariri in a Beirut bomb blast, is under international pressure to ensure that the legislative elections are not delayed.
"I say it from here that the hand is extended and the heart is open so that we all cooperate in the Lebanese interest"
Lawmaker Musbah Ahdab said Miqati, a wealthy businessman and a family friend of Syrian President Bashar al-Asad, was named because he pledged to meet opposition demands to hold the elections on time but not run himself, and to sack security chiefs and the public prosecutor.
Lebanon has been without a government since pro-Syrian prime minister Karami resigned at the end of February under the weight of huge protests sparked by al-Hariri's assassination.
On 10 March, Karami was again asked to form a new cabinet but threw in the towel on Wednesday, highlighting the difficulty of forming a government that appeases both pro-Syrian and opposition camps.
Karami's resignation fuelled charges that the pro-Syrian government was plotting to put off elections that the opposition is hoping to win.