A report claiming the elder McCullum brother had died was rubbished by the 38-year-old on Twitter as he uploaded a photo and declared himself "alive and kicking more than ever before".
The false report reached former Black Caps captain Brendon McCullum as he was flying back to New Zealand having competed in the T10 League in the United Arab Emirates.
He also dismissed the erroneous rumour regarding his brother on social media and promised to find the person who spread it.
"Tonight someone decided, via social media, to release that my brother passed away," Brendon McCullum wrote.
"I'm on a flight back to New Zealand and my heart broke! None of it is true! Whoever put this out there, I'll find you! Somewhere, somehow."
NZ Players Association chief executive Heath Mills said he had been contacted from cricketers in India who had heard the false reports and called Nathan McCullum to check.
"I have to say, my heart was racing before he picked up," Mills said.
He said Nathan was "very much alive" and had spent the day at a bowls event in Auckland.
