The Book of Common Prayer was first translated into Welsh during Queen Elizabeth I's reign, in 1567, by William Salesbury assisted by Richard Davies. Salesbury was "the most learned Welshman of his day", publishing the first Welsh - English dictionary and the first Welsh translation of the New Testament out of the Greek (also in 1567). Davies was Bishop of St. David's (1561-1581) and also aided Salesbury in the translation of the New Testament into Welsh.
Both this book and the Welsh New Testament were financed and published by Humphrey Toy (of Welsh origin) in London. The book appears in David Griffiths' Bibliography of the Book of Common Prayer as 192:2. It includes the entire Elizabethan (1559) Book of Common Prayer, less the Ordinal, but including the Psalter.
The book is presented here as PDF graphics and is taken from a reprint published in a limited edition of 300 copies by the Univ. of Wales Press (Gwasg Prifysgol Cymru) in 1965 (Griffiths 192:73). Besides the text of the Welsh Book of Common Prayer, the reprint includes a 41-page introduction in Welsh (not available here).