This paper presents the results of long-term observations (1964–2003) of direct solar radiation, to determine aerosol optical depth (AOD), made with a Linke-Feussner actinometer at the Tatra Mountain Meteorological Observatory on Mount Kasprowy Wierch (1991 m above sea level, 49.233°N, 19.982°E). To this end, broadband direct solar flux (0.29–2.9 µm) and wideband solar radiation measured with OG530 and RG630 filters are used to estimate the broadband and wideband (0.53–0.63 µm) AOD. The inversion algorithm used is based on the MODTRAN (MODerate resolution atmospheric TRANsmission) radiative transfer model applied to estimate direct flux for aerosol-free atmosphere. Total water vapor content, which accounts for the largest extinction of clear-sky direct flux, was obtained by radio sounding from the Poprad-Ganovce station (33 km from Mount Kasprowy Wierch) and from water vapor pressure measurements at the Observatory. The almost 900 clear-sky observations, performed close to noon time, found a significant long-term reduction of AOD. AOD decadal trends were −0.006 (−8 ± 4% [2σ]) with a 95% confidence interval of ± 0.003 and −0.014 (−13 ± 4% [2σ]) with a 95% confidence interval of ± 0.004 for broadband and wideband, respectively. Similar trends, but for years with negligible contamination of volcanic aerosol, are −0.012 (−16 ± 6% [2σ]) and −0.018 (−17 ± 6% [2σ]) with a 95% confidence interval of ± 0.003 and ± 0.004. However, positive AOD trends (from 0 to 0.04 per decade) were found between 1964 and 1983 and negative AOD trends (from −0.016 to −0.035 per decade) were found between 1984 and 2003. Changes of the AOD trends between both periods are associated with global dimming and brightening phenomenon, which took place in the second half of the twentieth century and at the beginning of the 21st century. The long-term mean broadband and wideband AOD were 0.07 ± 0.01 and 0.11 ± 0.02, respectively. Both quantities show a significant annual cycle, with maximum AOD during spring and summer. Very low values during the autumn and winter (about 0.03 for broadband and 0.05–0.06 for wideband) are the result of planetary boundary layer height (defined for the lower elevations), which during this period lies below Mount Kasprowy Wierch itself. The other explanation of low AOD values can be the high frequency (more than 50%) of inversion occurrence below the top of the mountain.