Background: The Y Balance Test–Upper Quarter (YBT–UQ) is a cost-effective, well-established, closed kinetic chain test to assess inter-limb asymmetries in the upper quarter that could negatively affect swimming performance. Thus, the aim of the present study was to determine YBT–UQ performances and inter-limb differences as well as its association with swimming performance in athletes with diverging levels of expertise.

Methods: Forty female and male competitive swimmers (age range: 10–22 years) with different expertise levels (A-squad: n = 9, B-squad: n = 12, C-squad: n = 19) were tested (reach distances for the YBT–UQ) and swimming performance was calculated using the ratio of individual to world best time.

Results: YBT–UQ performances (i.e., inferolateral reach direction for the dominant arm: p = .027, η_p²= .12 and the non-dominant arm: p = .031, η_p²= .17) but not YBT–UQ inter-limb differences significantly differed between groups and were largest in swimmers with the lowest expertise level (i.e., C-squad). Further, YBT–UQ performances (i.e., inferolateral reach direction [r = −.68 to −.70, both p < .05] and composite score [r = −.65 to −.67, both p < .05] for both arms and medial reach direction for the non-dominant arm [r = −.64, p < .05]) but not inter-limb differences were significantly and negatively correlated with swimming performance among B-squad swimmers.

Conclusions: Our results suggest that inter-limb differences in upper quarter mobility/stability are not influenced by the level of expertise and have no significant associations with swimming performance. However, greater reach distances were correlated with lower swimming performance for the B-squad swimmers indicating that a training-related increase in upper quarter mobility/stability could worsen swimming performance in those athletes.