A novel romaine lettuce cultivar, designated Green Thunder, is disclosed. The invention relates to the seeds of lettuce cultivar Green Thunder, to the plants of lettuce cultivar Green Thunder and to methods for producing a lettuce plant by crossing the cultivar Green Thunder with itself or another lettuce cultivar. The invention further relates to methods for producing a lettuce plant containing in its genetic material one or more transgenes and to the transgenic plants produced by that method and to methods for producing other lettuce cultivars derived from the cultivar Green Thunder.