In plants, there is a complex interaction between carbon (C) and nitrogen (N) metabolism and its coordination is fundamental for plant growth and development. In the present work, the influence of thioredoxin (Trx) m on C and N partitioning was studied using tobacco plants overexpressing Trx m from the chloroplast genome. The transgenic plants showed altered metabolism of C (lower leaf starch and soluble sugar accumulation) and N (with higher amounts of amino acids and soluble protein), which pointed to an activation of N metabolism at the expense of carbohydrates. To further delineate the effect of Trx m overexpression, metabolomic and enzymatic analyses were performed on these plants, indicating an up-regulation of the GS-GOGAT pathway. Trx m-overexpressing plants specifically displayed increased activity and stability of glutamine synthetase in tobacco plants. Moreover, higher photorespiration and nitrate accumulation were determined in these plants relative to the untransformed control, indicating that overexpression of Trx m favors the photorespiratory N cycle rather than primary nitrate assimilation. Taken together, the combined results reveal the importance of Trx m as a molecular mediator of N metabolism in plant chloroplasts.