A metal–organic framework Al-MIL-53-NHb>2b>-derived Brønsted acid catalyst (Al-MIL-53-RSOb>3b>H) has been synthesized employing a post-synthetic modification strategy under mild conditions. The Al-MIL-53-RSOb>3b>H catalyst was successfully utilized in the nitro-Mannich reaction taking advantage of its strong Brønsted acidity. Good to excellent yields of Mannich adducts were achieved for a variety of acylimine substrates in the presence of 0.1 mol% Al-MIL-53-RSOb>3b>H. Furthermore, the Al-MIL-53-RSOb>3b>H catalyst can be recycled five times without decreasing the yield and selectivity of Mannich adducts.