According to the invention, there is provided a novel romaine lettuce cultivar, designated Prima Forest. ‘Prima Forest’ is described as a vigorous romaine cultivar recommended for the main lettuce growing regions of California and Arizona. It has light plant weight, small core diameter, short plant height and also has resistance to lettuce downy mildew. This invention thus relates to the seeds of lettuce cultivar Prima Forest, to the plants of lettuce cultivar Prima Forest, to plant parts of lettuce cultivar Prima Forest, to methods for producing a lettuce cultivar by crossing the lettuce cultivar Prima Forest 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.