Electret-based microelectromechanical system (MEMS) vibratory energy harvesting is a key technology for converting the mechanical energy of environmental vibrations into electricity. Unfortunately, conventional electret charging methods generally rely on high-voltage and high-temperature processes that present limitations to MEMS design and production. Here, we show a MEMS post-processed self-assembled electret (SAE) that enables the integration of electrets with MEMS vibratory devices via evaporation as a post-MEMS process. Owing to the spontaneous orientation of polar molecules, the surface potential of the SAE can build up at room temperature in a microscopic region without charging processes, which enhances the design and fabrication flexibility of electret-based MEMS energy harvesters. We develop a MEMS vibratory device followed by post-processing the SAE and confirm induced electrical currents caused by the electrical field of the SAE at the vibrational input. This SAE-based MEMS technology is a promising design guideline for highly integrated single-chip MEMS vibratory energy harvesters. Published under an exclusive license by AIP Publishing.