Fanfiction has encountered problems with intellectual property law due to usage of copyrighted characters without the original creator or copyright owner's consent.

United States copyright law Significant amounts of copyrightable creative works such as motion pictures, television programs, music, and computer gaming works are produced in the United States. In addition, a significant amount of fanfiction is created in the United States. For these reasons, although every nation's law is different and different laws may apply to different works of fanfiction, U.S. law is often centrally relevant when determining the legality of writing and/or sharing fanfiction.

Under U.S. copyright law, the legality of a given work of fanfiction will depend principally on three legal doctrines: (1) copyrightability of the underlying source work; (2) the derivative work right; and (3) fair use.

To have copyright protection under U.S. law, a work must be an "original [work] of authorship fixed in any tangible medium of expression . . . from which [it] can be perceived, reproduced, or otherwise communicated, either directly or with the aid of a machine or device."17 U.S.C. §102(a)(2009). Such works of authorship include but are not limited to literature, music, plays, pictures and architectural works. Copyright cannot be applied to ideas, concepts, facts or other broad principles regardless of whether they are expressed in a tangible medium or otherwise. Copyright goes into effect automatically, even if a work is not published.17 U.S.C. §104(a)(b)(2009). For works created in 1978 or later, copyright protection persists for the life of the author plus 70 years; in the case of an anonymous work, a pseudonymous work, or a work made for hire, the copyright endures for a term of 95 years from publication, or for 120 years from the year of its creation, whichever expires first.17 U.S.C. §302(1998).

According to current United States copyright, copyright owners have the exclusive right "to prepare derivative works based upon [their] copyrighted work."17 U.S.C. §106(2)(2009). In the case where a copyright owner chooses to exercise their exclusive right to prepare derivative works against a work of fanfiction, they can sue the fanfiction writer for copyright infringement. To prove infringement, an owner must present evidence establishing that the accused has copied protected elements of the original work. If proven, possible infringement remedies include an order to cease sharing and/or to destroy the work (known as an injunction), or monetary damages. The remedy is dependent on the harm done to the copyright owner, the intent of the infringing person, and the grievousness of the infringement.17 U.S.C. §502 - 504(2009). An example of injunction as remedy was seen in the case of Anderson v. Stallone. There, Sylvester Stallone successfully pursued an action for copyright infringement against Anderson, an author who wrote a proposed script for Rocky IV, by proving that the copyright-protected characters used in the previous Rocky movies were central to the new script. The court enjoined Anderson from pursuing the creation of a movie or other published work based on his script.

Fanfiction will not be held liable for copyright infringement if it falls under the fair use defense. In determining the applicability of the fair use defense to a secondary use such as fanfiction, courts consider the following four factors: "the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes; the nature of the copyrighted work; the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and the effect of the use upon the potential market for or value of the copyrighted work."17 U.S.C. §107(2009).

Fair use is assessed on a case-by-case basis. While there are no bright-line rules, such genres as parody and criticism are enumerated by statute and case law as presumptively fair uses. There has been no case law that squarely addresses fanfiction in relation to fair use. Works of fanfiction are more likely to constitute fair use if they are "transformative" with respect to the original work, if they are non-commercial, if they appropriate relatively little of the original work, and/or if they do not tend to detract from the potential market for or value of the original work.

In a 2009 case, United States District Court judge Deborah A. Batts permanently prohibited publication in the United States of a book by Swedish writer Fredrik Colting, whose protagonist is a 76-year-old version of Holden Caulfield of J.D. Salinger's The Catcher in the Rye. Judge Batts explicitly rejected arguments of parody and criticism, stating, To the extent Defendants contend that 60 Years and the character of Mr. C direct parodic comment or criticism at Catcher or Holden Caulfield, as opposed to Salinger himself, the Court finds such contentions to be post-hoc rationalizations employed through vague generalizations about the alleged naivety of the original, rather than reasonably perceivable parody.

The case was vacated and remanded by the U.S. Court of Appeals for the Second Circuit with orders to apply the eBay v. MercExchange test in determining whether publication of a work can be prohibited on a theory of intellectual property infringement before the case has gone to trial.Salinger v. Colting, Docket No. 09-2878-cv The case was settled in 2011, with Colting agreeing to cease distribution.

In contrast, in Suntrust v. Houghton Mifflin Co., the United States Court of Appeals for the Eleventh Circuit vacated a temporary restraining order and preliminary injunction sought by the copyright holders of Margaret Mitchell's Gone with the Wind against Alice Randall's The Wind Done Gone. In determining whether Randall's work rose to the level of transformative, Circuit Judge Birch used the guidelines for transformative works laid out in the Supreme Court's Campbell v. Acuff Rose Music. Birch found Randall's work to be transformative because it "[provided] social benefit, by shedding light on an earlier work, and, in the process, creating a new one." Campbell had already established that the greater the transformative value a work held, the less important the other factors in the fair use test became. Despite Randall and Houghton Mifflin having released The Wind Done Gone as a commercial work, and Randall having used a substantial portion of Mitchell's work in her own, Birch found that the highly transformative nature of Randall's book overcame the other prongs of the fair use test.

Trademark law Separate from the legal issues raised by fanfiction's interaction with copyright law, legal issues may also develop from United States trademark law.

Current federal trademark law follows the Lanham Act, otherwise known as the Trademark Act of 1946. Under the Lanham Act, a trademark is "any word, term, name, symbol, or device, or any combination thereof" used in commerce to identify a service or good.U.S.C. § 1125 (2010) Under this definition, it is possible for the names and likenesses of television, film and book characters, fictional accounts, settings, or other elements of entertainment products to act as trademarks. Unlike copyright, however, trademark rights are not automatic. To establish a right in trademark, the rights-seeker must establish that their mark acts as a distinctive "source identifier" for a particular type of good or service. Thus, trademark rights may arise when a fictional character's name or likeness may serve to identify the source of an entertainment product or related good. For example, the use of Mickey Mouse's name or likeness may serve to identify a particular book or toy as originating from Disney. One way to establish that a mark acts as a distinctive source identifier is to establish that the relevant purchasing public has developed a strong association between the mark and its originating source. In legal terms, this is known as "secondary meaning."

If the trademark holder can show that its creation acts as a distinctive source identifier, they still must prove a likelihood of confusion to prevail in a trademark infringement claim. Different courts consider similar but not identical factors when deciding likelihood of confusion. Common factors that may be relevant to fanfiction include:

How well known and distinctive the mark allegedly being infringed is; How similar the infringing mark is to the original mark; How similar the allegedly infringing goods or services are to the markholder's goods or services; Whether the infringer intended to deceive the purchasing public or to trade on the good will of the markholder; The level of sophistication of those persons or groups likely to be the consumers of the mark; Whether consumers were actually confused as to the source of the goods or services.

The courts can weigh the factors in individual cases, and may consider additional factors as they please.

To the extent that fanfiction uses source-identifying characters, settings and such, the marks are often well known are identical to the original, and are used in similar types of goods (i.e., written fiction). In this way, the first three factors relayed here weigh for the trademark holder.

However, fanfiction writers generally do not intend to deceive the consuming public as to the source of the work, and often include prominent disclaimers at the outset of their works stating that the works are not the products of the original creators, both to honor the original creator and to prevent any possible confusion as to source. In addition, as a consuming audience, fanfiction readers are generally sophisticated regarding works' status as fanfiction, and are aware that fanfiction is not written or endorsed by those who hold the trademarks.Rebecca Tushnet, "Legal Fictions: Copyright, Fan Fiction, and a New Common Law" , 17 Loy. L.A. Ent. L.J. 651 (1997) As such, the last three factors tend to weigh in the direction of fanfiction writers.

Trademark holders may also allege that the use of trademarked characters, settings, etc. may constitute trademark dilution. The concept of trademark dilution is that overuse or improper use of a mark, even when it does not create consumer confusion, can lessen the mark's uniqueness and value as a source identifier. A dilution claim requires that the mark in question be famous throughout general consuming public and that the use of the mark create a likelihood of either "blurring" or "tarnishment." A likelihood of blurring occurs when the use of the mark creates an association that is likely to impair the distinctiveness of the famous mark; a likelihood of tarnishment occurs when the use of the mark creates an association that is likely to harm the reputation of the famous mark.

Even if a likelihood of confusion or dilution were found, trademark law provides various defenses to alleged infringement. These defenses fall into the categories of "fair use" and "First Amendment."