It covers 233 urban bodies spread across 26 districts, including five municipal corporations of Bareilly, Saharanpur, Jhansi, Firozabad and Moradabad, an official said.
Voting for 76 Nagar Palika Parishads and 152 urban panchayats is also underway.
The final phase is of major importance for the seven-month-old Yogi Adityanath government as 12 Cabinet Ministers and ten state Ministers hail from these areas where polling is taking place on Wednesday.
The Chief Minister has himself led the high-decibel campaign of the Bharatiya Janata Party, along with state unit chief Mahendra Nath Pandey.
The BJP has enjoyed a majority in most of these urban bodies and this time, the elections are being seen as the first litmus test of the Yogi Adityanath-led BJP government after its massive victory in state assembly polls held earlier in 2017.
Of the urban body polls held in 2012, BJP had a mayor in 12 of the 14 municipal corporations. Only Allahabad and Rampur had mayors from the opposition camp.
In 2012, Uttar Pradesh was under a Samajwadi Party (SP) rule and the feat was a major one for the BJP as it had not been in power for more than a decade then.
Thereafter, the BJP has been making impressive electoral gains and won 73 out of the 80 parliamentary seats in the watershed 2014 Lok Sabha elections. It then romped home with a decisive majority of 325 seats of the 403 assembly seats in the 2017 state assembly polls.
It is for the first time that all the other major parties - Bahujan Samaj Party (BSP), Samajwadi Party and Congress are contesting the urban body polls on their party symbols.
The SP, which was ousted by the BJP in the assembly polls is giving the ruling party a run for its money in many districts like Mahoba, Mainpuri, Jhansi, Etawah, Chandauli, Jaunpur, Kushinagar, Bareilly, Etah and Kannauj.