Josh Wardle, a software engineer in Brooklyn, knew his partner loved word games, so he created a guessing game for just the two of them. As a play on his last name, he named it Wordle.

But after the couple played for months, and after it rapidly became an obsession in his family’s WhatsApp group once he introduced it to relatives, Mr. Wardle thought he might be on to something and released it to the rest of the world in October.

On Nov. 1, 90 people played.

On Sunday, just over two months later, more than 300,000 people played.

It’s been a meteoric rise for the once-a-day game, which invites players to guess a five-letter word in a similar manner as the guess-the-color game Mastermind. After guessing a five-letter word, the game tells you whether any of your letters are in the secret word and whether they are in the correct place. You have six tries to get it right.

Few such popular corners of the internet are as low-frills as the website, which Mr. Wardle built himself as a side project. There are no ads or flashing banners; no windows pop up or ask for money. There is merely the game on a black background.

“I think people kind of appreciate that there’s this thing online that’s just fun,” Mr. Wardle said in an interview on Monday. “It’s not trying to do anything shady with your data or your eyeballs. It’s just a game that’s fun.”

This is not Mr. Wardle’s first brush with suddenly capturing widespread attention. Formerly a software engineer for Reddit, he created two collaborative social experiments on the site, called The Button and Place, that each were phenomena in their moment.

But Wordle was built without a team of engineers. It was just him and his partner, Palak Shah, killing time during a pandemic.

Mr. Wardle said he first created a similar prototype in 2013, but his friends were unimpressed and he scrapped the idea. In 2020, he and Ms. Shah “got really into” the New York Times Spelling Bee and the daily crossword, “so I wanted to come up with a game that she would enjoy,” he said.

The breakthrough, he said, was limiting players to one game per day. That enforced a sense of scarcity, which he said was partially inspired by the Spelling Bee, which leaves people wanting more, he said.

Word games have proved immensely popular for The Times and other companies in recent years, and many such as the Spelling Bee have developed devoted fan followings.

But since Wordle was built originally for just Mr. Wardle and Ms. Shah, the initial design ignored a lot of the growth-hacking features that are virtually expected of games in the current era. While other games send notifications to your phone hoping you’ll come back throughout the day, Wordle doesn’t want an intense relationship.

“It’s something that encourages you to spend three minutes a day,” he said. “And that’s it. Like, it doesn’t want any more of your time than that.”

Wordle lacked the ability to share results until mid-December. Mr. Wardle noticed players sharing their results by typing out a grid of green, yellow and black emojis, so he built an automated way for players to brag about their successes in a spoiler-free way.

If he were optimizing the game to gain as many players as possible, he would have included a link at the end of the tweet that the tool generates, he said. But after looking into it, he said it would have looked “trashy” and not as visually compelling, and he liked the grid’s mysterious air, which he felt piqued people’s interest.

While Ms. Shah was the lucky recipient of the first game, she has played a key role in getting it ready for the public, Mr. Wardle said. An initial list of all of the five-letter words in the English language — about 12,000 — contained a lot of obscure words that would have been near impossible to guess.

So he created another game for Ms. Shah: This time, she would sort through those 12,000 or so words, designating whether or not she knew them. That narrowed down the list of Wordle words to about 2,500, which should last for a few years. (Already, a few words have riled up the fans: Some were upset by REBUS and TAPIR, saying they were not familiar enough.)

Ms. Shah says she wakes up every day with a new routine: She warms up with the Spelling Bee, which gets her mind right for Wordle. She also loves the New York Times Crossword and cryptic crosswords.

Though Wordle is now shared with the world, she said she appreciated that Mr. Wardle originally created it for her.