Boateng, 38, secured a shock loan move from Italian side Sassuolo to Barcelona in 2019, making just four appearances during his time at the club.
The deal reportedly had an €8 million option to buy included, but Barcelona ultimately decided against activating it.
Now, speaking on podcast Unscripted, Boateng has revealed that the transfer only went ahead once Messi gave his blessing.
"At the time, there were two sporting directors,” he said.
“One of them was Eric Abidal, and he said, 'Yes, we want you.' The other sporting director said, 'We want you'. The president (Bartomeu) wanted me, the coach (Valverde) too, so I said, 'Perfect, let's sign tomorrow!' They replied, 'No, we have to talk to Leo first.'
“He had that power. I went to sleep hoping Messi would approve my signing. If he had refused, I wouldn't have signed for Barcelona."