Not only did indigenous Americans have sign language, but one particular sign language dominated an incredibly vast field of space, crossing many language families and cultural zones, in a unified form that, even if they could not speak one another's languages, there would be no error or difficulty communicating so long as they used sign language. This particular one is commonly called Plains Indian Sign Language or Plains Sign Talk. I shall refer to it as PST for the remainder of the post and for the sake of ease.

PST had, at its peak, an incredibly vast range of fluent speakers. It was viewed as not just a language for the deaf, but a means of conducting intercontinental trade. In some ways, it is a linguistic remnant of indigenous long-distance trade from as far as the Pacific Northwest and the coast of Texas. Most non-Inuit nations west of the Mississippi are well-attested in using PST. Wikicommons provides this lovely map, showing which groups particularly, overlaid on top of North America, were well attested in the history of its use. PST is represented in the purple. This was used by several Algic peoples (such as the Oji-Cree, Arapaho, and Cheyenne), Uto-Aztecans (such as the Comanche, Pima, or Hopi), Caddoan (such as the Wichita or Pawnee), Dene (such as the Navajo and Apache), Siouan (such as the Lakota, Omaha, or Mandan), Tanoan (Taos and Kiowa), Sahaptian (Nez Perce and Sahaptin), Irquoisan (Huron), as well as 'isolate' families such as the Salish, Zuni, Kutenai, Cayuse, and the so-called "Coahuiltecan" languages like Karankawa, Tonkawa, and various Coahuiltec-proper languages. That is an incredibly wide spread of completely unrelated languages to use a single, common sign language, that is a version of none of them, but instead an entirely unique language in its own right. Here is a video of various tribal representatives communicating in PST, recorded in the 1930s.

Also seen on the map are a few other forms. While most of these are European inventions, particularly around the Eastern seaboard, there are a few other indigenous ones seen. The Plateau Sign Language, for instance, existed once, but through the action of indigenous peoples such as the Crow was replaced with the more common PST. Inuit Sign Language makes an appearance as well, in the far north a light blue strip. Although only attested in Nunavut, it may have once been more widespread, potentially into Greenland or Alaska across most of the Eskaleutian family. Also present on the map is Hawaiian Sign Language, first attested by outside observers in 1821, and once the dominant form of sign language in the archipelago. That Hawaii is a Polynesian culture, but a relatively isolated one, means that my grouping it among indigenous American languages is something of a technicality - it is more closely related to other South Pacific or even Indonesian groups than it is to what we'd generally consider Native American, and Hawaii's isolation makes it unlikely that their particular sign language existed elsewhere in Polynesia.

What's not on the map, is that there are more sign languages known from the south of the US. The Maya have their own sign language, distinct from national varieties in the countries in which they live, with commonality but also perhaps dialectal difference between lowland (Yucatec) and highland (K'iche) forms. It is known more properly by the Maya as "Mute language", meant to convey without words, rather than necessarily being a language for deaf use.

The concept of "mute language" plays an intriguing part in the story of indigenous sign languages. It would be reasonable to say that, even if unattested, many, perhaps even most, indigenous nations and tribes had a sign language of some sort - either spread across a wide area, or highly localized. In all of these cultures, from PST to the Maya, their sign language is not only used for trade, but also for important ceremonial and ritual purposes. Storytelling, the ancient custom of conveying myths and legends, frequently makes use of sign language as a key element. In this sense, the sign languages may not have been developed for deaf use at all, and most 'speakers' were not deaf - instead, the deaf use of these would be complimented by the widespread knowledge of the signs and would make their lives and social integration much easier almost by coincidence more than intention.

While there are some significant gaps in our understanding of indigenous American sign languages, it would not be safe to assume that just because a sign language is not attested in a particular area, that it did not have one. Absence of evidence is not evidence of absence, especially as more recent research begins to reveal that, if an environment with no suitable alternative exists, a sign language will begin to develop naturally out of necessity. While we may not know for sure the specific details, indigenous sign languages have generally played an important part in cultural expression, traditional lifestyles, and the passing and preservation of myths, legends, fables, stories, rituals, and ceremonies. It is ironic that, in such ceremonial use, the spoken word is often vocable rather than meaningful, while the motions made carry the entire story quite literally in the palm of their hands.