According to the invention, there is provided a novel romaine lettuce cultivar, designated Solid King. ‘Solid King’ is described as a vigorous romaine cultivar with large frame size and heavier weight, short core length, savoyed and glossy leaf color, corky root rot resistance, no fringe burn on mature leaves and also yellower heart leaf color with a solid mid rib. This invention thus relates to the seeds of lettuce cultivar Solid King, to the plants of lettuce cultivar Solid King, to plant parts of lettuce cultivar Solid King, to methods for producing a lettuce cultivar by crossing the lettuce cultivar Solid King with another lettuce cultivar, and to methods for producing a lettuce cultivar containing in its genetic material one or more backcross conversion traits or transgenes and to the backcross conversion lettuce plants and plant parts produced by those methods.