The ageing and recycling of polyamide 12 (PA12) powder are key issues in selective laser sintering (SLS) processing, while hygroscopicity is an important defect of PA12 products during use. This paper aims to correlate PA12 powder recycling with changes in the hygroscopicity of printed parts, which is significant for a comprehensive understanding of the impact of ageing on the performance of PA12 parts. PA12 powder was utilized in different grades: virgin, post-industrial recycled, and virgin-post-industrial recycled mixture (virgin-to-post-industrial recycled powder weight ratio of 25/75). The mutual effects of the powder grade, print orientation used in the building process, and absorbed moisture on the properties of printed parts were evaluated. Molar mass increase and secondary crystallization have been demonstrated in the post-industrial recycled powder, being responsible for the worse sintering quality in the SLS process. Therefore, the printed parts made from the post-industrial recycled powder or the virgin-post-industrial recycled powder mixture exhibited worse mechanical strength, a higher porosity, and hence a higher level of hygroscopicity in terms of moisture absorption and wettability. On the contrary, the printed parts made from the virgin powder were found to be more moisture-sensitive in terms of the decline in mechanical strength in the wet state because of the higher content of the amorphous phase holding the plasticizing water.