The party of the 65-year-old cricketer-turned-politician emerged as the single largest side in the National Assembly in the 25 July general elections.
Imran was nominated by PTI's parliamentary committee which met at a hotel here, DAWNNews TV reported. He left his Banigala residence for the first time after his party's victory in last week's elections.
Despite Imran's explicit instructions and refusal to travel with a large protocol, a number of cars and heavy security tagged along with him, the channel added.
While the exact date of the swearing-in of Pakistan's new prime minister has not been announced, reports say it could take place on 14 August, the Independence Day of the country.
Pakistan's National Assembly comprises a total of 342 members, of which 272 are directly elected.
A party can only form the government if it manages to clinch 172 seats in total. Allies and reserved seats will take the PTI tally to 174 seats in the National Assembly.