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.