Purpose: Glass ionomer cement (GIC) is a commonly used restorative material. However, it lacks sufficient strength and wear resistance to be used as a posterior restorative material. The purpose of this study was to evaluate the compressive and diametral tensile strength (DTS) of conventional restorative GIC reinforced with nanohydroxyapatite (nHAP) derived from the cockle shell (Anadara granosa). Materials and methods: A total of 72 specimens were prepared and divided into one control Group (Group I, n = 12 conventional restorative GIC) and five test Groups of specimens of GIC mixed with nHAP (n = 12 specimens each) in the following proportions (Group II-1%; Group III-2%; Group IV-4%; Group V-5%; Group VI-6%). Polytetrafluoroethylene (PTFE) mold with specifications of 6 mm height and 4 mm diameter was used to make samples for compressive strength (CS) testing. A mold dimension of 2 mm thick and 4 mm diameter was used for fabricating samples for DTS. The specimens were kept in simulated saliva for 7 and 15 days before testing. Compressive and DTS was evaluated under a universal testing machine. Kruskal–Wallis test was used to calculate inferential statistics, followed by the post hoc Nemenyi test. Wilcoxon signed-rank test was used to analyze samples from 7 to 15 days. Results: The setting time increased steadily with an increase in nHA concentration. The highest mean setting time value was found in Group VI (51.66 ± 6.83 minute), followed by Group V (23.6 ± 7.12 minute), Group IV (13.43 ± 0.49 minute), Group III (9 ± 0.89 minute), and Group II (7.5 ± 0.65 minute), respectively and the difference is statistically significant (0.001). CS decreased with an increase in the concentration of Anadara granosa-derived nHAP content in the composition of GIC. The highest mean value is seen in Group III (40.66 ± 7.28 MPa), followed by Group II (39.6 ± 2.06 MPa), Group IV (33.43 ± 6.26 MPa), Group V (19.33 ± 7.60 MPa), and Group VI (14.66 ± 6.47 MPa) and the difference is statistically significant (0.001). DTS initially decreased with 1 and 2% addition of nHAP, but later increased at 6%. The highest mean value is seen in Group III (55 ± 4.97 MPa), followed by Group II (44.33 ± 10.44 MPa), Group IV (38.43 ± 0.89 MPa), Group VI (31.3 ± 7.17 MPa), and Group V (19.66 ± 5.46 MPa) and the difference is statistically significant (0.001). Conclusion: The CS of nHAP-incorporated samples decreases steadily with an increase in the concentration of reinforcement additive to conventional GIC. Diametral tensile strength initially decreased with 1 and 2% addition of nHAP but later increased to 6%. The samples immersed in saliva for 15 days showed more increase in CS and DTS.