Warrior women are women who are trained in the arts of warfare and take part in battle, warfare etc.
In Westeros male warriors are plentiful. Westerosi warrior women on the other hand are quite uncommon. Westerosi women however are not forbidden from becoming warriors but neither is it encouraged or seen as acceptable and feminine by many Westerosi. Nor is a woman viewed as a suitable opponent for men, especially during one-to-one combat. Even hitting a shield with the image of a woman can make a man feel unchivalrous.
Despite Westeros having a martial culture, a culture that reveres the warrior, no separate warrior class in exists for women in Westeros, except in the ranks of the Poor Fellows, a military order under the command of the High Septon. They accepted women as warriors.
One of the few famous warrior women in Westeros’s histories is Visenya Targaryen, who wielded the Valyrian steel sword Dark Sister and rode Vhagar into battle. Unlike her sister Visenya Targaryen there is no mention of Rhaenys wielding a sword or any other weapon but like her sister she commanded armies and rode her dragon into battle during the War of Conquest. According to George R. R. Martin both women were warriors and dragonriders in their own rights.
In Westeros women tend to be considered the gentler sex, in need of protection. The ability for a woman to be able to defend herself is considered unnecessary and unfeminine besides. Highborn women are not encouraged towards warfare or expected to know how to physically defend themselves, although they may perhaps be instructed on the tactics of warfare, especially if they are female heirs to a House. Lord Randyll Tarly sums up many a man’s thought on women warriors,
The gods made men to fight, and women to bear children. A woman’s war is in the birthing bed.
It is not possible for a woman to become a knight, nor is there a female equivalent of knighthood for a woman who is bent on living a martial life, for a woman who also wishes to abide by a chivalric code. Although Brienne of Tarth is as skilled as most any a knight there is no place among them for a woman such as she. Nor is there a feminine title for a woman warrior, such as Ser as knights have, to acknowledge her ability and chosen vocation, much to the confusion of Podrick Payne.
Further north however it is not so unusual for a woman to be able to physically defend herself. The women of House Mormont and Bear Island have had no choice but to learn how to preserve themselves during attacks from ironmen and wildling raiders while their menfolk are absent. Lady Maege Mormont tells Catelyn Stark,
Beyond the Wall there are spearwives, they are accepted and respected by the men of the Free Folk. The Free Folk culture is a warrior culture, one that gives women the right, but not the obligation, to be fighters.  The same applies to the mountain clansmen of the Mountains of the Moon, who are also a First Men tribal culture, like the Free Folk; Chella, daughter of Cheyk, is a woman warchief of her own clan.
The ironborn are more also more accepting of women warriors and are willing to fight alongside them.
In the ancient nation of the Patrimony of Hyrkoon there was possibly a custom of having warrior women. A present there is a culture for warrior woman east of the Bone Mountains which may originate from the Patrimony. There are warrior maids of Kayakayanaya, Bayasabhad and Shamyriana who fight bare-breasted and pierce their nipples with iron rings and cheeks with rubies. Lomas Longstrider apparently visited these successor cities as he said that there are no fiercer fighters in the world than their warrior women.
The greatest of the Sarnori went to battle in scythed chariots pulled by teams of bloodred horses. These were often driven by their wives or daughters, for it was the custom among the Tall Men for men and women to make war together.
It is not known if the Valyrians had warrior women. Thus far there has been no mention of Dark Sister being passed on to a female Targaryen after Visenya. There is, however, ample evidence for the existence of female dragonlords both in ancient Valyria and among the Targaryens.
Warrior Women of Westeros
- Visenya Targaryen – famously wielded Dark Sister.
- Brienne of Tarth – wields Oathkeeper
- Asha Greyjoy – fights with two throwing axes in either hand
- Arya Stark – wields Needle
- The women of Bear Island
- Hagen's beautiful red-haired daughter
- Obara Sand - wields a spear and whip.
- Meris - sellsword of the Windblown
- Danelle Lothston - the "Mad Lady" of Harrenhal, who personally commanded her men-at-arms and wore armor during the Second Blackfyre Rebellion
Warrior Women Beyond the Wall
- Ygritte - spearwife
- Harma Dogshead 
- Holly - spearwife
- Rowan - spearwife
- Squirrel - spearwife
- Frenya - spearwife
- Myrtle - spearwife
- Willow Witch-eye - spearwife
Warrior Women of Essos
- Barsena, pit-fighter, who had slain every other woman sent against her.
- warrior maids of Bayasabhad
- warrior maids of Shamyriana
- warrior maids of Kayakayanaya
- jhattar Zhea
- some jhats and jhattars of the Jogos Nhai
- Sarnori warrior women
Warrior Women of the Summer Islands
All men must die, and women too... but not all will be remembered.
It is a rare and precious gift to be a knight.
Men will always underestimate you and their pride will make them want to vanquish you quickly, lest it be said that a woman tried them sorely.
Her last foe was a northman with an axe, a big man bald and bearded, clad in a byrnie of patched and rusted mail that could only mean he was a chief or champion. He was not pleased to find himself fighting a woman.—Asha Greyjoy's thoughts
I wanted to present my female characters in great diversity, even in a society as sexist and patriarchal as the Seven Kingdoms of Westeros. Women would find different roles and different personalities, so women with different talents would find ways to work with it in a society according to who they are. 
She is supposed to be freakish. She was an answer to the bad fantasy cliché of warrior women.
- A Clash of Kings, Chapter 22, Catelyn II.
- A Feast for Crows, Chapter 33, Jaime V, p 606.
- The Citadel. “Land Ownership and Marriage in Westeros?”So Spake Martin
- December 24, 2005, More Targaryen Descriptions So Spake Martin
- Dornish women - So Spake Martin
- A Storm of Swords, Chapter 45, Catelyn V, p 522.
- June 03, 2003, Concerning Wildling Women. So Spake Martin
- Dornish women - So Spake Martin
- The World of Ice & Fire, The Bones and Beyond.
- The World of Ice & Fire, The Grasslands.
- The Mystery Knight.
- June 03, 2003, Concerning Wildling Women.So Spake Martin
- A Dance with Dragons, Chapter 11, Daenerys II.
- A Storm of Swords, Chapter 11, Jaime II.
- A Feast for Crows, Chapter 4, Brienne I.
- A Feast for Crows, Chapter 14, Brienne III.
- A Dance with Dragons, Chapter 26, The Wayward Bride.
- A Dance with Dragons, Chapter 42, The King's Prize.
- George R.R. Martin: "Trying to please everyone is a horrible mistake" Adria's News
- November 18, 2005, US Signing Tour (Half Moon Bay, CA) So Spake Martin