Is Boston lettuce the same as butter lettuce?

Yes. Boston lettuce is a kind of butter lettuce: it is one of the two butterhead subtypes grown in the United States, and the other is Bibb.

Butterhead cultivars form round heads filled with leaves, much like iceberg, but their leaves are generally thin with a pliable, oily texture instead of being thick and crisp. Within the group, the Boston type is larger and lighter green than the Bibb type, so a head sold as Boston lettuce is the bigger, paler form of butterhead. Butterhead, leaf and Latin lettuces all mature more quickly than romaine, while crisphead (iceberg) lettuce takes about 60 to 130 days after planting. In Europe, butterheads are grouped differently, into summer outdoor types and winter types, and the summer types are larger. So when a recipe calls for butter lettuce, either Boston or Bibb matches the description, with Boston giving you larger, lighter leaves.

Source: Hayes, R. J., “Lettuce Production,” Compendium of Lettuce Diseases and Pests, 2nd ed., American Phytopathological Society (reproduced by California Leafy Greens Research Program)

Image: Buttercrunch butterhead lettuce, by Geo Lightspeed7, CC BY-SA 4.0, via Wikimedia Commons.

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