HBpep-SA is a cell-permeable peptide coacervate that undergoes phase separation to form stable microdroplets at a pH below 6.5. It efficiently delivers a range of macromolecules, from therapeutic peptides as small as 726 Da to large enzymes up to 430 kDa. The loaded coacervate remains stable under conditions near physiological and serum levels until cellular internalization occurs. HBpep-SA is applicable for intracellular delivery of large macromolecules and can serve as a carrier for mRNA-based vaccines.