An adjustable device holder includes a holder base including a bottom panel, a cover panel covering the bottom panel, two parallel sliding grooves located on the bottom panel and a shaft column located at the bottom panel between the sliding grooves, and an adjustment module including two adjustment plates slidably coupled to the sliding grooves and having a respective integrated tooth rack, two clamping members respectively pivotally connected to the adjustment plates for holding a mobile electronic device therebetween and a rotating wheel coupled to the shaft column of the holder base and having an integrated external ring gear meshed between the tooth racks of the adjustment plates.