Messi is contracted until 2018, but speculation over his future has intensified with talks over a new deal seemingly progressing slowly.
"Leo will renew," the Brazil international told beIN Sports.
"That is for sure."
Neymar was the hero for Barca as they claimed a 6-1 win over PSG to record a 6-5 aggregate victory in the last 16.
He scored a late brace and set up Sergi Roberto's decisive 95th-minute goal, while he also won a penalty converted by Messi in the first half.