On‐skin electronics that offer revolutionary capabilities in personalized diagnosis, therapeutics, and human–machine interfaces require seamless integration between the skin and electronics. A common question remains whether an ideal interface can be introduced to directly bridge thin‐film electronics with the soft skin, allowing the skin to breathe freely and the skin‐integrated electronics to function stably. Here, an ever‐thinnest hydrogel is reported that is compliant to the glyphic lines and subtle minutiae on the skin without forming air gaps, produced by a facile cold‐lamination method. The hydrogels exhibit high water‐vapor permeability, allowing nearly unimpeded transepidermal water loss and free breathing of the skin underneath. Hydrogel‐interfaced flexible (opto)electronics without causing skin irritation or accelerated device performance deterioration are demonstrated. The long‐term applicability is recorded for over one week. With combined features of extreme mechanical compliance, high permeability, and biocompatibility, the ultrathin hydrogel interface promotes the general applicability of skin‐integrated electronics.