The anterior thalamic nuclei (ATN) are a crucial processing hub for memory and neural network integration. We explored the influence of the ATN in combination with state-of-the-art temporary optogenetic inhibition of its projections to medial prefrontal cortex (mPFC) and dorsal subiculum (dSUB) in spatial memory using the Win-Shift Eight-Arm Radial Arm Maze (RAM). We also assessed c-fos, an immediate early gene, expression changes in anterior cingulate (ACC) and rhinal cortices in these interconnected regions. We used 28 PVG hooded rats in four groups: bilateral ATN lesions (ATN Bilat), unilateral viral vector infusion to inhibit cortical axon terminals and ATN lesion (VV+ATN), unilateral viral vector infusion and Sham lesion (VV+Sham), and a control non-opsin viral vector infusion and ATN lesion (VVneg+ATN). We found that only rats in the ATN Bilat group made significantly more errors in the RAM, while the other three groups were all unimpaired. Interestingly, the ATN Bilat group with no viral vector infusion showed increased errors during optogenetic stimulation, potentially indicating hypersensitivity to the optic-fibre light shining. Ex vivo, c-fos expression was significantly increased in the viral vector infused hemisphere of ACC for VV+Sham and VVneg+ATN groups - a puzzling finding given the VVneg+ATN group received the no-opsin (control) infusion. However, c-fos expression was similar between ACC hemispheres for VV+ATN rats, aligning with prior studies. While unexpected, these findings demonstrate the ATN is crucial for spatial memory and suggest light may influence ATN retinal inputs, which warrants further investigation.