Two-dimensional material-based p–n junctions are widely used in nano- and microelectronic devices. Compared to conventional doping methods, surface-charge-transfer doping provides a reliable, simple, and nondestructive approach to modulating carrier properties of 2D materials. However, despite its advantages, this method has not been used to form p–n junctions for thermoelectric applications. This paper introduces a lateral p–n homojunction temperature sensor, fabricated via simple on-sheet chemical doping of a transition metal dichalcogenide (TMDC) nanosheet grown by chemical vapor deposition. While five-layer PtSe2 is semimetallic, area-selective surface doping with benzyl viologen and Magic Blue is used to suppress ambipolar transport and define distinct n-type and p-type regions. The resulting Seebeck coefficient difference between the two regions enables sensitive detection of temperature gradients, with a resolution of ∼0.1 K. This doping-based approach avoids complex processing and structural damage, offering both high sensitivity and fabrication simplicity. Our method offers a scalable route for fabricating p–n homojunctions in 2D materials, and can thus be employed to develop self-powered, high-resolution temperature sensors for a broad range of applications, from chip-scale devices to biomedical applications.