Display omitted •Sensitive and amplification-free microRNA (miRNA) detection using a portable electrochemiluminescence biosensor.•Electrochemiluminescence sensing of miRNA with a mobile phone camera.•A rapid electrochemiluminescence sensor for miRNA‑21 detection combining a molecular beacon with magnetic extraction. microRNAs (miRNAs) have emerged as important biomarkers for numerous diseases, but their widespread use has been hampered by the lack of point-of-care testing solutions. Herein, we demonstrate a sensitive, rapid and portable electrochemiluminescence (ECL) based sensor for miRNA-21 combining a switch-on ECL molecular beacon with magnetic bead based extraction of the miRNA target sequence. Streptavidin coated magnetic beads were functionalised with a hairpin molecular beacon; one stem of the beacon was functionalised with an ECL active label and the opposing stem was functionalised with a quencher. When the target miRNA was present, the hairpin opened, separating the label and quencher, generating an intense ECL signal. The sensor provides a simple and robust strategy for miRNA detection without the need for intercalating labels, reporter probes for detection, added enzymes or hairpin primers for target amplification. The detection limit of the sensor is 500 attomoles of miRNA-21 when using a photomultiplier tube for ECL detection. The potential of the developed sensor for point-of-care applications has been demonstrated by combining the molecular beacon ECL sensor with a portable potentiostat for ECL generation and a mobile phone camera for ECL detection of miRNA-21.