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Thermal tuning of the optical refractive index in the waveguides to control light phase accumulation is essential in photonic integrated systems and applications. In silicon photonics, microheaters are mainly realized by metal wires or highly doped silicon lines placed at a safe distance (1um) from the waveguide to avoid considerable optical loss. However, this poses a significant limitation for heating efficiency because of the excessive free-carrier loss when a heater is brought closer to the optical path. In this work, we present a new concept of using optically transparent 2D semiconductors (e.g. MoS2) for realizing highly efficient waveguide integrated heaters operating at telecom wavelengths. We demonstrate that a single-layer MoS2 heater with negligible optical absorption in the infrared can be placed in close proximity (only 30nm) to the waveguide and show the best-reported heating efficiency of 15 mW per FSR without sacrificing the optical insertion loss. The heater response time is 25us, limited by Au 1L-MoS2 Schottky contact. Both the efficiency and response time can be further significantly improved by realizing 2D MoS2 heaters with ohmic contacts. Our work shows clear advantages of employing 2D semiconductors for heaters applications and paves the way for developing novel energy-efficient, lossless 2D heaters for on-chip photonic integrated circuits.