Telomerase　has　long　been　considered　as　a　biomarker　for　cancer　diagnosis　and　a　therapeutic　target　for　drug　discovery.　Detecting　telomerase　activity　in　vivo　could　provide　more　direct　information　of　tumor　progression　and　response　to　drug　treatment,　which,　however,　is　hampered　by　the　lack　of　an　effective　probe　that　can　generate　an　output　signal　without　a　tissue　penetration　depth　limit.　In　this　study,　using　the　principle　of　distance-dependent　magnetic　resonance　tuning,　we　constructed　a　telomerase-activated　magnetic　resonance　imaging　probe　(TAMP)　by　connecting　superparamagnetic　ferroferric　oxide　nanoparticles　(SPFONs)　and　paramagnetic　Gd-DOTA　(Gd(III)　1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic　acid)　complexes　via　telomerase-responsive　DNA　motifs.　Upon　telomerase-catalyzed　extension　of　the　primer　in　TAMP,　Gd-DOTA-conjugated　oligonucleotides　can　be　liberated　from　the　surface　of　SPFONs　through　a　DNA　strand　displacement　reaction,　restoring　the　T1　signal　of　the　Gd-DOTA　for　a　direct　readout　of　the　telomerase　activity.　Here　we　show　that,　by　tracking　telomerase　activity,　this　probe　provides　consistent　monitoring　of　tumor　growth　kinetics　during　progression　and　in　response　to　drug　treatment　and　enables　in　situ　screening　of　telomerase　inhibitors　in　whole-animal　models.　This　study　provides　an　alternative　toolkit　for　cancer　diagnosis,　treatment　response　assessment,　and　anticancer　drug　screening.　©　2023　American　Chemical　Society.