Heparin is an indispensable drug in anticoagulation therapy but with a narrow therapeutic window, which dictates regular testing and dose adjustment. However, current monitoring tools have a long turnaround time or are operator intensive. In this work, we describe a cellulose-based photoacoustic sensor for heparin. The sensors have a turnaround time of 6 min for whole blood samples and 3 min for plasma samples regardless of heparin concentration. These sensors have a limit of detection of 0.28 U/ml heparin in human plasma and 0.29 U/ml in whole blood with a linear response (Pearson's r = 0.99) from 0 to 2 U/ml heparin in plasma and blood samples. The relative standard deviation was < 12.5% in plasma and < 17.5% in whole blood. This approach was validated with heparin-spiked whole human blood and had a linear correlation with the activated partial thromboplastin time (aPTT) (r = 0.99). We then studied 16 sets of clinical samples-these had a linear correlation with the activated clotting time (ACT) (Pearson's r = 0.86, P < 0.0001). The photoacoustic signal was also validated against the cumulative heparin dose (Pearson's r = 0.71, P < 0.0001). This approach could have applications in bed-side heparin assays for continuous heparin monitoring.