Acute myocardial infarction (AMI), also recognized as a "heart attack," is one leading cause of death globally, and cardiac myoglobin (cMb), an important cardiac biomarker, is used for the early assessment of AMI. This paper presents an ultrasensitive, label-free electrochemical aptamer-based sensor (aptasensor) for cMb detection using polyethylenimine (PEI)-functionalized reduced graphene oxide (PEI-rGO) thin films. PEI, a cationic polymer, was used as a reducing agent for graphene oxide (GO), providing highly positive charges on the rGO surface and allowing direct immobilization of negatively charged single-strand DNA aptamers against cMb via electrostatic interaction without any linker or coupling chemistry. The presence of cMb was detected on Mb aptamer-modified electrodes using differential pulse voltammetry via measuring the current change due to the direct electron transfer between the electrodes and cMb proteins (Fe3+/Fe2+). The limits of detection were 0.97 pg mL-1 (phosphate-buffered saline) and 2.1 pg mL-1 (10-fold-diluted human serum), with a linear behavior with logarithmic cMb concentration. The specificity and reproducibility of the aptasensors were also examined. This electrochemical aptasensor using polymer-modified rGO shows potential for the early assessment of cMb in point-of-care testing applications.