A new fabrication method for an ultrathin (500 nm thick) pressure-sensitive adhesive (PSA) was demonstrated by utilizing a series of in situ cross-linked viscoelastic copolymer films. Viscoelastic films composed of poly(2-hydroxyethyl acrylate-co-2-ethylhexyl acrylate) were synthesized successfully in a one-step manner by an initiated chemical vapor deposition (iCVD) process, where free-radical polymerization is triggered in the vapor phase either by heat or UV, or a combination of both. In particular, the photoinitiated chemical vapor deposition method generated a highly cross-linked polymer film, whereas cross-linking of the copolymer film was suppressed greatly in the conventional thermal iCVD method. A combination of thermal and photoinitiated chemical vapor deposition could regulate the cross-linking density of the copolymer films. We controlled the cross-linking density of the copolymer films to exhibit a viscoelastic property so that they would readily adhere to various kinds of substrates with only SOO nm thick copolymer PSA. The adhesion performance of the PSA was systematically optimized by tuning the copolymer composition as well as the cross-linking density, and consequently a high shear strength of more than 85.2 +/- 5 N/cm(2) was achieved despite the 500 nm thickness. In addition, the PSA was completely transparent. We expect that the ultrathin PSAs developed in this work will be utilized widely for the realization of various soft electronic devices, which usually require strong adhesion, tunable viscoelastic properties, and optical transparency.