threads was generated by secreting liquid mussel foot proteins (Mfps) from mussel foot. These Mfps are assembled and manufactured by glands by injection molding reaction. [3] The foot of a mussel presses against the surface to create a vacuum chamber, which propels the delivery of fluidic Mfps. It is believed that Mfps confined to plaques, such as Mfp-2, Mfp-3, Mfp-4, and Mfp-5, create coacervates when exposed to saltwater. All mfps include the post-translational amino acid DOPA, and mfp-5 contains the largest concentration of DOPA residues (30 mol%) and leads to strong adhesion. [4] It was reported that the coacervation of mfps, which occurs in a variety of ways, such as complex coacervation driven by electrostatic interaction, as revealed in polyions of Mfp-131 and Mfp-151, [5] and self-coacervation driven by electrostatic and/or hydrophobic forces as revealed in Mfp-3S. [6] An essential component in mussel adhesion is the special amino acid L-DOPA with its catechol group. [7] The catechols act as chemical jack-of-all-trades or Janus-like chemicals and allow attachment to almost all types of material surfaces using either covalent or noncovalent bonds. [8] Several research groups have suggested mussel-inspired materials to be used as bio-inspired adhesives, [9] with efforts being made by including features of the mussel foot proteins in synthetic polymers. [8a,b,10] Hence, catechols and analogues thereof are highly relevant in materials design. [8e,g] Great strides have been made in harnessing mussel-inspired chemistry in materials, however, many of previous efforts lack a simple and efficient way to deliver the materials to a surface underwater for efficient adhesion. [11] The blue mussel is thought to deliver its adhesive in the form of complex fluids that spread spontaneously and exhibit strong reversible interfacial bonding and tunable cross-linking. [4] The complex fluids are coacervates [12] -mainly composing mixtures of polyelectrolytes that form a separate phase from the aqueous medium. Thus, the mussel maintains its complex fluid glue that upon meeting the surface, spreads and presents DOPA residues to the target surface for attachment and subsequent curing (Figure 1A,B). Indeed, coacervates are used widely in biology (Figure 1A), including in sandcastle worm adhesives [13] and to infiltrate squid beak scaffold materials with proteins. [14] In the blue mussel, the whole gluing process takes only a few minutes and occurs within the spatial confines of the byssus Adhesion underwater is a major challenge. Mussel-inspired complex coacervates functionalized with L-3,4-dihydroxyphenylalanine (L-DOPA) are proposed for underwater adhesives through versatile chemistry of DOPA, however, simple, efficient, controllable, and nontoxic procedures to harness them are still under investigation. In this study, inspired from the mussel byssus formation process, coacervate adhesives are formed underwater by simple injection of an acidic proto-coacervate of DOPA functionalized polyelectrolytes on underwater surface...