Observations of rotational modulations of brown dwarfs and giant exoplanets allow the characterization of condensate cloud properties . As of now rotational spectral modulations have only been seen in three L-type brown dwarfs . We report here the discovery of rotational spectral modulations in LP261-75B , an L6-type intermediate surface gravity companion to an M4.5 star . As a part of the Cloud Atlas Treasury program we acquired time-resolved Wide Field Camera 3 grism spectroscopy ( 1.1–1.69 \mu m ) of LP261-75B . We find gray spectral variations with the relative amplitude displaying only a weak wavelength dependence and no evidence for lower-amplitude modulations in the 1.4 \mu m water band than in the adjacent continuum . The likely rotational modulation period is 4.78 \pm 0.95 h , although the rotational phase is not well sampled . The minimum relative amplitude in the white light curve measured over the whole wavelength range is 2.41 \pm 0.14 % . We report an unusual light curve with seemingly three peaks approximately evenly distributed in rotational phase . The spectral modulations suggests that the upper atmosphere cloud properties in LP261-75B are similar to two other mid-L dwarfs of typical infrared colors , but differ from that of the extremely red L-dwarf WISE0047 .