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Background: Myocardial fibrosis is closely associated with cardiac dysfunction, arrhythmia, and sudden cardiac death. A model of myocardial fibrosis induced by isoprenaline was used to determine the influence of exogenous adiponectin on myocardial fibrosis. Materials and Methods: A total of 30 mice were equally and randomly divided into three groups as follows: isoproterenol (ISO, n = 10), adiponectin and isoproterenol (APN + ISO, n = 10), and sham (n = 10). ISO (7.5 mg/kg/day) was injected intraperitoneally (i. p.) in the ISO group and APN + ISO group for three consecutive days to establish a model of myocardial fibrosis; normal saline was injected in the sham group. From day 4, adiponectin was injected (10 μg/kg/day; i. p.) in the APN + ISO group for 32 days; normal saline was injected in ISO and sham groups for 32 days. Five weeks later, mice were sacrificed, and myocardial tissue was dyed by Sirius Red to observe the collagen morphology. The collagen volume fraction was calculated. Protein and messenger RNA (mRNA) expression of type-I and type-III collagen in the myocardium was detected by reverse transcription-polymerase chain reaction and western blotting, respectively. Results: Type-I collagen in myocardial interstitial tissue was significantly increased in the ISO group. Collagen in myocardial interstitial tissue in the ISO + APN group was decreased significantly compared with the ISO group. Expression of type-I collagen mRNA in ISO and ISO + APN groups was significantly higher than that in the sham group (3.01 ± 0.78, 2.32 ± 0.82 vs. 1, respectively; P < 0.01). Expression of type-I collagen mRNA in the ISO + APN group was significantly lower than that in the ISO group (2.32 ± 0.82 vs. 3.01 ± 0.78, respectively; P < 0.01). Expression of type-III collagen mRNA in the ISO group, and ISO + APN group was significantly higher than that in the sham group (2.39 ± 0.70, 1.62 ± 0.57 vs. 1, respectively; P < 0.01). Expression of type-III collagen mRNA in the ISO + APN group was significantly lower than that in the ISO group (1.62 ± 0.57 vs. 2.39 ± 0.70, respectively; P < 0.01). Expression of type-I collagen protein in the myocardium in the ISO + APN group was significantly higher than that in the sham group (0.73 ± 0.19 vs. 0.28 ± 0.15, respectively; P < 0.001), but it was significantly lower than the expression of type-I collagen protein in the ISO group (0.53 ± 0.17 vs. 0.73 ± 0.19, respectively; P < 0.001). Conclusions: Supplementation with exogenous adiponectin can inhibit myocardial fibrosis induced by ISO.