A study is presented of H i line observations of the nearby spiral galaxy NGC 3521 observed with the VLA as part of The H i Nearby Galaxy Survey . Clearly evident in the H i data cube is the presence of an anomalous H i component that is both diffuse and slow-rotating . The data cube is dynamically decomposed into regular and anomalous H i components . A mass of \mathrm { M _ { HI } } = 1.5 \times 10 ^ { 9 } M _ { \odot } is estimated for the anomalous H i - 20 per cent of the total H i mass . Standard H i data products and rotation curves are produced for each dynamical component . In terms of circular rotation speed , the anomalous H i is found to lag the regular H i by \sim 25 - 125 km s ^ { -1 } . Three-dimensional models are generated and used to determine the possible location of the anomalous H i . The results strongly suggest it to be distributed in a thick disc with a scale-height of a few kpc ( \sim 3.5 kpc ) . It is concluded that the anomalous H i in NGC 3521 constitutes a slow-rotating halo gas component , consistent with similar findings for other nearby galaxies . A study of the radial distribution of the anomalous H i shows it to be spatially coincident with the inner regions of the stellar disc where the star formation rate is highest . It is most likely a galactic fountain that has deposited gas from the disc of the galaxy into the halo .