He replaced the former LoP and ex-Congressman Radhakrishna Vikhe-Patil who left the party to join the Bharatiya Janata Party (BJP) and is now a Cabinet Minister in Chief Minister Devendra Fadnavis' government.
Last week, Wadettiwar - who became the first legislator from the Vidarbha region to be appointed as the LoP - was named the Congress group leader in the Assembly for which allies Nationalist Congress Party and other parties extended their unanimous support.
Fadnavis, who also hails from Nagpur (in Vidarbha) and had described Wadettiwar as a future Leader of Opposition last week, on Monday welcomed his appointment and lauded his long years of public service in the eastern part of the state.
Vikhe-Patil and other leaders congratulated Wadettiwar on his new post.
Elected from Brahmapuri and a former minister, Wadettiwar (57) has been rewarded with the post - enjoying a Cabinet Minister status - for preventing the Congress's total elimination in the state after the May Lok Sabha elections.
Appointed in-charge of Chandrapur and Gadchiroli districts, he managed to get the Shiv Sena's Warora legislator Suresh Dhanorkar into the Congress' fold, barely weeks before the elections.
Dhanorkar was given the Congress ticket for Chandrapur and he emerged a giant killer - defeating BJP strongman and Union Minister of State for Home Hansraj Ahir - and helping the opposition party to secure its sole Lok Sabha seat from the state.
Wadettiwar is the third Leader of Opposition in the Maharashtra Assembly since 2014.
The post was first held by the Shiv Sena's Eknath Shinde briefly before the party joined the government in late 2014. It then went to the Congress' Vikhe-Patil till he left the party and joined the government last Sunday.
( With inputs from IANS )