The filmmaker and composer, along with legendary lyricist Vairamuthu, have been working together for 25 years.
Commenting on his long-standing relationship with Rahman, the "Roja" filmmaker told reporters here: "It's been 25 years, but it just feels like I met Rahman yesterday.
"Every film with Rahman is a special journey. He brings newness to each project, new sound and new music."
Ratnam was speaking at the audio launch of "Kaatru Veliyidai".
He said he is thankful for this collaboration with Rahman and Vairamuthi. The triumvirate's journey started with 1992 Tamil romantic drama "Roja".
Rahman said he's blessed to have found a mentor in Ratnam.
"I'm thankful to the platform I have received because of Mani sir. If Balachander sir had not introduced me to Mani sir, I wouldn't have had this platform," Rahman said.
Slated to release on April 7, "Kaatru Veliyidai" stars Karthi as a fighter pilot and Aditi Rao Hydari as a doctor.