BACKGROUND: Facet joint orientation (FJO) and facet joint tropism (FJT) are associated with intervertebral disc degeneration and paraspinal muscle atrophy. However, none of the previous studies has evaluated the association of FJO/FJT with fatty infiltration in the multifidus, erector spinae, and psoas muscles at all lumbar levels. In the present study, we aimed to analyze whether FJO and FJT were associated with fatty infiltration in the paraspinal muscles at any lumbar level. METHODS: Paraspinal muscles and FJO/FJT were evaluated from L1-L2 to L5-S1 intervertebral disc levels on T2-weighted axial lumbar spine magnetic resonance imaging. RESULTS: Facet joints were more sagittally and coronally oriented at the upper and lower lumbar levels, respectively. FJT was more obvious at lower lumbar levels. The FJT/FJO ratio was higher at upper lumbar levels. Patients with sagittally oriented facet joints at the L3-L4 and L4-L5 levels had fattier erector spinae and psoas muscles at the L4-L5 level. Patients with increased FJT at upper lumbar levels had fattier erector spinae and multifidus at lower lumbar levels. Patients with increased FJT at the L4-L5 level had less fatty infiltration in the erector spinae and psoas at the L2-L3 and L5-S1 levels, respectively. CONCLUSIONS: Sagittally oriented facet joints at lower lumbar levels could be associated with fattier erector spinae and psoas muscles at lower lumbar levels. The erector spinae at upper lumbar levels and psoas at lower lumbar levels might have become more active to compensate the FJT-induced instability at lower lumbar levels.