This research aimed to assess the clinical usefulness of serum kappa (κ) and lambda (λ) free light chains (FLCs) in patients with bladder cancer (BC). One hundred samples were collected and analysed from healthy volunteers (C) and bladder cancer patients. Cancer patients were divided into two subgroups: low-grade (LG) and high-grade cancer (HG). Concentrations of FLCs, CEA, CA19-9, creatinine and urea were measured per manufacturers’ guidelines. The concentrations of κ and λ FLCs and CEA were significantly higher in BC patients in comparison to the control group. Moreover, the concentrations of κ and λ FLCs and CEA were significantly higher in both low-grade as well as high-grade cancer in comparison to the controls. The levels of κ and λ FLCs differed between tumour grades, with patients presenting higher concentrations in high-grade compared to low-grade cancer. In the total study group, κFLC correlated with λFLC, the κ:λ ratio, CRP, CEA, CA19-9, creatinine and urea. There was also a correlation between λFLC and κFLC, CRP, CEA, creatinine and urea. The λFLC showed a higher ability (sensitivity and PPV) to detect bladder cancer in comparison to κFLC and CEA. In addition, λFLC had a higher ability to exclude BC (specificity and NPV) than κFLC and CEA. λFLC also showed the highest accuracy in the detection of bladder cancer. In conclusion, the revealed differences in the concentrations of both κ and λ FLCs suggest their potential participation in bladder cancer development. Increased concentrations of free light chains in bladder cancer patients and the association with the tumour grade suggest that κ and λ FLC measurements may be useful in the diagnosis and prognosis of bladder cancer. This is the first research that evaluates the concentration of FLCs in bladder cancer, so further studies are necessary to confirm their usefulness as tumour markers of this malignancy.