"Nice guy" is an informal term, commonly used with either a literal or a sarcastic meaning, for a man. In the literal sense, the term describes a man who is agreeable, gentle, compassionate, sensitive, and vulnerable. The term is used both positively and negatively. When used positively, and particularly when used as a preference or description by someone else, it is intended to imply a man who puts the needs of others before his own, avoids confrontations, does favors, provides emotional support, tries to stay out of trouble, and generally acts nicely towards others.

In the context of a relationship, it may also refer to traits of honesty, loyalty, romanticism, courtesy, and respect. When used negatively, a nice guy implies a man who is unassertive or otherwise unattractive. The opposite of a genuine "nice guy" is commonly described as a "jerk", a term for a mean, selfish and uncaring person. A man is labeled a "jerk" on how he treats his partner, seen as the extreme case where he would not have a sensitive or kind side and is seen as a "macho man" and insensitive type.

However, the term is also often used sarcastically, particularly in the context of dating, to describe someone who believes himself to possess genuine "nice guy" characteristics, even though he actually does not, and who uses acts of friendship and basic social etiquette with the ulterior aim of progressing to a romantic or sexual relationship. This is sometimes referred to as "nice guy syndrome", which is used to describe a sense of entitlement to sexual or romantic attention from women simply for being "nice," and irrational anger when that attention is not reciprocated. Although the "nice guy syndrome" is used primarily in the context of dating relationships, it can also apply to the workplace as well as politics.

In their qualitative analysis (1999), Herold and Milhausen found that women associate different qualities with the "nice guy" label:

"Some women offered flattering interpretations of the 'nice guy', characterizing him as committed, caring, and respectful of women. Some women, however, emphasized more negative aspects, considering the 'nice guy' to be boring, lacking confidence, and unattractive."

The "bad boys" were divided into two categories, "as either confident, attractive, sexy, and exciting or as manipulative, unfaithful, disrespectful of women, and interested only in sex".

This distinction helped further the understanding of why women might prefer "nice guys" or "not-nice guys". Women were asked for their preferences and what values they may look for in each relationship, such as attractiveness, and sexual desires in short- and long-term relationships.^{[further explanation needed]}

Nice guys are sometimes suggested to be overbearing or lacking in vision and ambitions; these opinions suggest self-confidence as a key-point and area of improvement. Often these ideas and views of a certain nice guy can contribute to a woman's willingness to pursue a romantic relationship.