Nanoplastics (<1000 nm) are gaining high attention worldwide as an emerging environmental contaminant because they are easier to be taken up by organisms and likely to pose higher ecological and health risks than microplastics (<5 mm). However, we are still lacking harmonized and reliable methodologies for analyzing nanoplastics in environments. Here, we reviewed 33 studies on state-of-art methodologies for pretreatment, separation, identification, and quantification of nanoplastics. Most of the studies successfully detected standard reference nanoplastics spiked in environmental samples but failed to separate and quantify nanoplastics from real field samples. Up to date, only five studies measured nanoplastics in real field samples, i.e., seawater, snow, air, sand, and agriculture soil samples, respectively. Raman spectroscopy and pyrolysis–gas chromatography/mass spectrometry (py-GC–MS) are the most popular analytical methods. However, the current spectroscopic methods are time-consuming and cannot cover the whole nano-range due to the detection limit of particle size; determination of the mass concentration of nanoplastics by the mass spectrometry methods are destructive, thus requiring extra/subsamples to obtain physical information of nanoplastics. The major caveat is that the quantification is often conducted without chemical confirmation of polymer types, raising concerns about the reliability of current results. It is also worth noting that recovery tests and blank controls, both of which are general steps in the quantification of conventional chemical pollutants, are rarely reported in nanoplastic studies. More efforts should be made to enhance the reliability and accuracy of nanoplastic analysis in environmental samples, which can only be achieved with strict chemical confirmation and adequate quality assurance along with the whole analytical process.