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Macrophages engulf "foreign" cells and particles, but phagocytosis of healthy cells and cancer cells is inhibited by expression of the ubiquitous membrane protein CD47 which binds SIRPα on macrophages to signal "self". Motivated by some clinical efficacy of anti-CD47 against liquid tumors and based on past studies of CD47-derived polypeptides on particles that inhibited phagocytosis of the particles, here we design soluble, multivalent peptides to bind and block SIRPα. Bivalent and tetravalent nano-Self peptides prove more potent ( K eff ∼ 10 nM) than monovalent 8-mers as agonists for phagocytosis of antibody opsonized cells, including cancer cells. Multivalent peptides also outcompete soluble CD47 binding to human macrophages, consistent with SIRPα binding, and the peptides suppress phosphotyrosine in macrophages, consistent with inhibition of SIRPα's "self" signaling. Peptides exhibit minimal folding, but functionality suggests an induced fit into SIRPα's binding pocket. Pre-clinical studies in mice indicate safety, with no anemia that typifies clinical infusions of anti-CD47. Multivalent nano-Self peptides thus constitute an alternative approach to promoting phagocytosis of "self", including cancer cells targeted clinically.