Reconstituted basement membrane products, like Matrigel, suffer from variability and xenogenic contaminants, hindering three-dimensional cell culture models. To overcome these challenges, we developed engineered basement membranes (eBMs) using peptide-conjugated alginate hydrogels with independently tunable mechanics. Ile-Lys-Val-Ala-Val (IKVAV)–modified eBMs, with fast stress relaxation and low stiffness, supported normal mammary acinus formation. Both increased stiffness and slow relaxation were required to induce invasion in IKVAV-modified eBMs, differing from the invasive phenotype observed in Arg-Gly-Asp (RGD)–modified eBMs regardless of the mechanical properties. Mechanistic studies revealed the balance of β1 and β4 integrin signaling, hemidesmosome formation, and laminin production were influenced by eBM properties. Inhibiting focal adhesion kinase or hemidesmosome signaling disrupted acinus formation in IKVAV-modified eBMs. This defined, xenogenic-free eBM system offers a modular platform for tissue engineering and disease modeling. A modular, tunable platform helps unravel cell-matrix interactions and drives tissue growth.