This study validated a German version of the Sport Motivation Scale (SMS28) and investigated the sex-specific and age-related differences in motivation of competitive mountain runners. Translation and cross-cultural adaptation of the SMS28 was based on translation and back-translation methodology. Acceptable validity of the German version of the SMS28 was indicated by the high correlations (.81 to .98) of scores on the seven subscales for the English and German versions completed by 15 subjects. Motivation analysis was performed with 127 competitive male and female mountain runners. The seven subscales of the German version showed good internal consistency (Cronbach's coefficient alphas .70 to .85). Findings on motivation of competitive mountain runners were a decline across age groups of Intrinsic motivation toward accomplishment for both sexes and an age-related decline of External regulation only for females. These motivational changes might well be associated with the observed diminishing numbers of older participants in mountain running competitions.