In　this　study,　manganese　tellurite　(MnTeO3)　nanoparticles　are　developed　as　theranostic　agents　for　magnetic　resonance　imaging　(MRI)-guided　photothermal　therapy　of　tumor.　MnTeO3　nanoparticles　are　synthesized　via　a　simple　one-step　method.　The　as-synthesized　MnTeO3　nanoparticles　with　uniform　size　show　good　biocompatibility.　In　particular,　MnTeO3　nanoparticles　exhibit　a　high　photothermal　conversion　efficiency　(η =　26.3%),　which　is　higher　than　that　of　gold　nanorods.　Moreover,　MnTeO3　nanoparticles　also　have　high　MRI　performance.　The　longitudinal　relaxivity　(r1)　value　of　MnTeO3　nanoparticles　is　determined　to　be　8.08　±　0.2　mm−1　s−1,　which　is　higher　than　that　of　clinically　approved　T1-contrast　agents　Gd-DTPA　(4.49　±　0.1　mm−1　s−1).　The　subsequent　MnTeO3　nanoparticles-mediated　photothermal　therapy　displays　a　highly　efficient　ablation　of　tumor　cells　both　in　vitro　and　in　vivo　with　negligible　toxicity.　It　is　demonstrated　that　MnTeO3　nanoparticles　can　serve　as　promising　theranostic　agents　with　great　potentials　for　MRI-guided　photothermal　therapy.　©　2021　Wiley-VCH　GmbH