The Church of Jesus Christ of Latter Day Saints (commonly called
the "Mormon" Church) discontinued the practice of polygamy nearly
120 years ago, in 1890.
Anyone who entered into a polygamous relationship after this
date was excommunicated. This practice of excommunication of
polygamists continues today.
To read the official declaration from Church President Wilford
Woodruff which outlawed the practice of polygamy in the Church,
please see the "Related Link" below.
While the above answer is accurate, it leaves some interesting
information out. When President Wilford Woodruff discontinued
polygamy in 1890 a schism occurred within the Church. A small group
of men felt they had been ordained by Woodruff's predecessor, John
Taylor, to continue the practice of polygamy even if the main body
of the Church did not. As a result they broke off from the Church
and formed other congregations that are commonly referred to by the
media with the common nickname, "Mormon." But in fact they have no
connection to the LDS Church headquartered at Temple Square in Salt
Lake City, Utah. Today, the remnants of those groups are found in
the Fundamentalist Church of Jesus Christ of Latter Day Saints
(FLDS Church)--popular due to the recent Warren Jeff's trial in
Texas--, the Apostolic United Brethren, and other Mormon
However, there were instances that families and individuals who
remained with the larger body of the LDS Church continued to
practice polygamy. Some members of the Quorum of the Twelve even
continued to perform polygamous marriages. For that reason Joseph
F. Smith, successor to Woodruff as President of the Church, issued
the "Second Manifesto" in 1904. President Heber J. Grant followed
up with the "third and fourth Manifestos" that were intended to
reiterate the ideals taught in 1890 by President Woodruff. But to
answer your question the official date of the end of polygamy
within the LDS Church was October 6, 1890.
Note: see Anne Wilde, "Fundamentalist Mormonism: Its History,
Diversity and Stereotypes, 1886-Present," in Scattering of the
Saints: Schism within Mormonism, edited by Newell G. Bringhurst and
John C. Hammer (Independence, MO: John Whitmer Books, 2007),
The principal division of Mormonism, the Church of Jesus Christ
of Latter Day Saints (LDS), has never actually abandoned or
repudiated the doctrine of plural marriage. The doctrine
itself was not reversed, revoked or otherwise invalidated by the
1890 Manifesto of Wilford Woodruff, then-President and "Prophet" of
the LDS church.
In FACT, the doctrine and practice of plural marriage were only
"SUSPENDED" by the LDS on the basis of a rationale that obliged
members to obey civil law, the particular civil law being the law
against polygamy. It thus follows that, should that law be
rescinded, Mormons would once more become subject to the "new and
everlasting covenant" of plural marriage prescribed by Joseph
Smith, Jr. in the Doctrine & Covenants, chapter 132.
Those questioning this are referred to an official publication
of the LDS church, "Articles of Faith," by "Apostle" James E.
Talmage, one of the most esteemed (at least by Mormons) of Mormon
theologians. For many years, his book was a standard reference
source carried by Mormon missionaries in their ubiquitous
backpacks. The subtitle of the book is "Being a Consideration of
the Principal Doctrines of the Church of Jesus Christ of Latter-day
Saints" It is published by the LDS's own publishing house, Deseret
From page 384 of the 1984 edition:
"An illustration of such suspension of divine law is found in
the action of the church regarding the issue of plural
A "suspension" is not a nullification or a reversal. The
doctrine of plural marriage still stands; it has merely been
"suspended." It would have been inexpedient to revoke the
doctrine, seeing that the "Prophet" Joseph Smith, Jr. had boldly
declared it to be a "new and everlasting covenant." Something fully
abandoned after only a few decades could scarcely be said to be
"everlasting." Faced with the need to get away from plural marriage
and into statehood, the LDS circumvented the implications of the
"everlasting" descriptor and adopted the expedient of "suspension."
Should the courts of this nation ever hold that plural marriage is
legally valid, the doctrine presumably would return in full force
and effect and the LDS branch of Mormonism could than join their
maverick cousins, the "Fundamentalist Mormons" in the practice of
the "new and everlasting covenant."