It is the latest honour to be bestowed upon him since his record-breaking triple century for New Zealand against India at the city's Basin Reserve ground this week.
Wellington mayor Celia Wade-Brown offered 32-year-old McCullum the "Keys to the City" in a letter after the series-sealing innings on Tuesday.
McCullum's mammoth test score of 302 was the highest by a New Zealand batman, beating Martin Crowe's 299 against Sri Lanka, again at the Basin, in 1991.
It's the first time the mayor has offered the ceremonial keys to the city since 2002, when Lord of the Rings' director Sir Peter Jackson and scriptwriter Fran Walsh accepted their keys, along with Weta Workshop's Richard Taylor and Tania Rodger.
Mayor Celia Wade-Brown said the award was a fitting recognition of McCullum's achievement.
"Wellingtonians and the cricket world congratulate Brendon McCullum on his magnificent achievement," she said.
"The Basin Reserve is rightly regarded worldwide as a special Test venue and has hosted many triumphs and dramatic moments.
"Brendon's magnificent innings will be remembered for decades to come by the thousands of people at the Basin, and the tens of thousands of others who stopped everything in the middle of a working day to witness sports history being made."
She said the honour will ensure McCullum, who lives in Christchurch, will always be "at home" in the city of his greatest sporting performance.
McCullum said he was "honoured and humbled by the gesture".
He indicated he will accept the keys a ceremony, expected to be in June.
After helping win the 2005 Ashes series, English talisman Andrew 'Freddie' Flintoff was awarded the freedom of his home city of Preston.
It was said that the honour gave him the ancient privilege of being allowed to herd flocks of sheep through the town.
- By Kurt Bayer of APNZ
