Autophagy is fundamental for organisms to survive under stressful conditions. While activated, it mediates the degradation and recycling of cytoplasmic materials through vacuolar degradation in plants and yeast, and lysosomal degradation in mammalian cells. However, the feedback regulation mechanism for autophagy remains largely unclear. The energy sensor SNF1-related protein kinase 1 (SnRK1) has been well established as an upstream player to activate autophagy via phosphorylation of several autophagic machineries. Surprisingly, we identified SH3 domain-containing protein (SH3P2), a plant-unique adaptor protein which binds to autophagy-related 8 (ATG8), as a downstream substrate of SnRK1.. Utilizing biochemical assays, we show that SH3P2 is phosphorylated by SnRK1 complex. Overexpression of the phosphomimic SH3P2 enhances autophagic activity but reduces plant survivability, whereas SnRK1 knockdown suppresses SH3P2 activity in autophagy. Moreover, upregulated phosphorylation in SH3P2 impaired the endocytic pathway, indicating that SnRK1-mediated phosphorylation functions as a molecular switch for SH3P2 dynamics in autophagy and the endocytic pathway.