Poor posture during desk-based learning activities can lead to many health issues, such as spinal problems, musculoskeletal discomfort, and myopia. While traditional posture correction systems use immediate feedback to notify users of their wrong posture, they often disrupt users’ concentration. This study explores an ambient projection notification system for non-intrusive posture correction notifications during desk-based learning scenarios. Through a notification elicitation study and an expert co-design workshop, we investigated users’ perception towards basic elements of projection notification and derived a design space for desk-based projection notification. We then implemented and evaluated PosProjector, an ambient projection notification system, by applying two notification strategies that embody the most representative dimensions of the design space. Results showed that PosProjector can improve users’ posture with little task interference and support various media, including paper and tablets. We further discussed the implications of how to design the least intrusive projection notification system for posture correction.