We report on the design, fabrication, and characterization of an integrated detector for electron spin resonance spectroscopy operating at 27 GHz. The microsystem, consisting of an LC-oscillator and a frequency division module, is integrated onto a single silicon chip using a conventional complementary metal-oxide-semiconductor technology. The achieved room temperature spin sensitivity is about 10(8) spins/G Hz(1/2), with a sensitive volume of about (100 mu m)(3). Operation at 77 K is also demonstrated. (C) 2012 Elsevier Inc. All rights reserved.