P21 (disambiguation)

P21, p21 or P-21 may refer to:

• The p21/waf1 protein, also known as cyclin-dependent kinase inhibitor 1 or CDK-interacting protein 1
• The p21/ras protein, part of the RAt Sarcoma family of proteins
• Curtiss XP-21, an American fighter aircraft of the 1920s
• LÉ Emer (P21), a patrol vessel of the Irish Navy
• The tax form P21 accompanies tax refunds in Ireland.