Hello, hope you are doing well!

LIST OF REQUIRED DOCUMENTS-

For NITs, GFTIs and IITs-

• Three Passport Size Photographs identical to JEE Main 2020 Registration
• Provisional Seat Allotment Letter
• Candidate Undertaking
• Proof of Fee Payment by e-challan or Internet Banking of SBI for Seat Acceptance (Rs. 15, 000 for SC/ST/PwD candidates and Rs. 35,000 for others)
• JEE Main 2020 Score Card
• JEE Main 2020 Admit Card
• 10th Standard Mark Sheet or Date of Birth Proof
• 12th Standard (or equivalent) Mark Sheet and Pass Certificate
• Photo Identity Card
• Medical Certificate
• Category Certificate (if applicable)
• PwD Certificate (if applicable)
• OCI Certificate or PIO Card (if applicable)
• Print out of Filled Choices for Seat Allotment

For IITs-

• Provisional Seat Allotment Letter
• Two Passport Size Photographs identical to JEE Advanced 2020 registration
• Undertaking by the Candidate
• Proof of Seat Acceptance Fee Payment by e-challan or net banking of SBI (Rs. 15, 000 for SC/ST/PwD candidates and Rs. 35,000 for others)
• Valid Photo Identity Card
• JEE Advanced 2020 Admit Card
• Date of Birth Proof (10th Standard Mark Sheet)
• 12th Standard (or equivalent) Mark Sheet and Pass Certificate
• Medical Certificate
• Category Certificate (if applicable)
• Certificate for PwD (if applicable)
• Passport (for foreign nationals) of candidates & parents or OCI Certificate or PIO Card (if applicable)
• DS Certificate (if applicable)
• Registration cum Locked Choices for Seat Allotment

So, yes 12th marksheet is needed at the time of JOSAA counselling.