The effect of cognitive effort on mood is unclear. Expending cognitive effort is generally avoided but expenditure of cognitive effort is the primary feature of some popular recreational activities, such as sudoku or video games. However, one common confound is that previous studies mostly looked into mood changes before and after cognitive effort manipulation. Therefore, it is unknown what im- mediate impact cognitive effort has on momentary mood. To investigate this, in two studies, we used a letter sorting paradigm to test whether momentary mood ratings change with different levels of experimentally manipulated task difficulty. In study 1 (N = 105), we found that increased difficulty, and consequently cogni- tive effort, leads to more errors and lower mood ratings. In study 2 (N = 210), we replicated our results from study 1 and further demonstrated that cognitive effort influences mood independently from immediate reward. In conclusion, controlling for reward conditions, we found that cognitive effort reduces people’s moment to moment mood.