“Port Arthuran” is the name of an inhabitant of Port Arthur, Texas. The name “Port Arthuran” has been cited in print since at least 1927.
   
“Port Arthurian” has been cited in print since at least 1922, but is used with much less frequency. The Port Arthur (TX) News has used both “Port Arthurian” and “Port Arthuran,” but “Port Arthuran” has been its preferred name. “Arthurian” is usually used relating to the British king Arthur.
   
 
Wikipedia: Port Arthur, Texas
Port Arthur is a city in Jefferson County within the Beaumont–Port Arthur Metropolitan Statistical Area of the U.S. state of Texas. The population was 57,755 at the 2000 census.
 
3 March 1922, Port Arthur (TX) Daily News, pg. 1, col. 7:
Port Arthurians will meet the new city engineer at the chamber dinner Tuesday.
 
6 April 1922, Port Arthur (TX) Daily News, pg. 1, col. 1:
Port Arthurians will get a chance to hear all about the girls’ camp fires at Franklin auditorium Friday afternoon at 4 o’clock when Miss Edith M. Kempthorne, national field camp fire secretary, will speak.
   
12 February 1927, Port Arthur (TX) News, pg. 7, col. 1:
“BLOSSOM TIME”
AUDIENCE IS BIG
Romantic Operetta Again
Thrills Port Arthurans
 
29 April 1927, Port Arthur (TX) News, pg. 12, col. 2:
FORMER PORT ARTHURAN
DIES IN SAN ANTONIO
 
OCLC WorldCat record
The Port Arthuran.
Publisher: [Port Arthur, Tex. : Gulf Coast Publishers]
Edition/Format:  Journal, magazine : Periodical : English
 
OCLC WorldCat record
Conversations with Port Arthurans.
Author: Port Arthur Independent School District.
Publisher: Port Arthur, Tex. : Port Arthur Independent School District, 1974.
Edition/Format:  Book : English
 
4 October 1974, Port Arthur (TX) News, “Supper Table Talk” by John Ayres, pg. 5, col. 1:
Residents here are Port Arthurans, Beaumonters go by that tag and our Big H. neighbor’s folks are Houstonians—and the Nederland C. of C. officially designates its hometowners as Nederlanders.
 
Port Arthur (TX) News
April 11, 2011
PAPL Newsnotes: History of Port Arthur
Darragh Doiron
PORT ARTHUR — The history of Port Arthur is to be recounted through the eyes of environmental and cultural historian Chris Sellers, associate professor of history at the State University of New York at Stony Brook.
(...)
Chris is also interviewing several long-time Port Arthurians to learn of their experiences of living in an environment largely defined by major refineries.
 
Port Arthur (TX) News
January 4, 2012
Christina Christi Knutsen Dolce
(...)
A Port Arthuran her whole life, Christi was born on December 23, 1935, the only child of Rolf Knutsen and Clara Smith Knutsen, who both preceded her in death in 1977.