Purpose The purpose of this study is to investigate the role of motivation for knowledge sharing (KS) by assessing how four qualitatively different motivation types, as per self-determination theory (SDT), predict KS, its quality and its undesirable counterpart, knowledge withholding. Design/methodology/approach The study was carried out as a survey (n = 200) in an expert organization. The analyses were conducted using structural equation modeling. Findings Autonomous type of extrinsic motivation (identified motivation) was the strongest predictor of KS (in work meetings) and its quality, whereas the other motivation types (intrinsic, introjected and external) had no independent contribution to variance in KS. Knowledge withholding was negatively associated with identified and positively with external KS motivation. Research limitations/implications Single organization limits the generalizability of the results. Future studies should further investigate the role of identified motivation for various KS behaviors. Practical implications The findings suggest that autonomy-supportive management practices known to facilitate self-determined behavior can improve KS. Fostering external motivation by incentivizing KS may be both ineffective and have undesirable consequences. Originality/value Few prior studies investigate KS motivation beyond external and intrinsic motivation or apply SDT to KS using SDT-based scales. This study distinguishes between four different motivation types and is the first to investigate their differential impact on KS and its quality. It is also the first to demonstrate the importance of identified motivation for KS. It further elucidates how the quality of KS motivation is reflected in knowledge withholding, an overall underinvestigated behavior.