Peptides are one of the most important functional motifs for constructing smart drug delivery systems (DDSs). Functional peptides can be conjugated with drugs or carriers via covalent bonds, or assembled into DDSs via supramolecular forces, which enables the DDSs to acquire desired functions such as targeting and/or environmental responsiveness. In this mini review, we first introduce the different types of functional peptides that are commonly used for constructing DDSs, and we highlight representative strategies for designing smart DDSs by using functional peptides in the past few years. We also state the challenges of peptide-based DDSs and come up with prospects.