A cooktop appliance and griddle assembly are generally provided herein. The cooktop appliance may include a top panel, a heating element attached to the top panel, an upper plate, and a lower plate. The upper plate may be disposed above the top panel along a vertical direction. An upper plate may have a top cooking surface and a bottom heating surface. The top cooking surface may extend perpendicular to the vertical direction to receive a cooking item thereon. The bottom heating surface may be positioned beneath the upper plate and face the top panel to receive a thermal output from the heating element. The lower plate may extend perpendicular to the vertical direction beneath a portion of the upper plate and above the top panel. A radiation channel may be defined between the lower plate and the bottom heating surface along the vertical direction.