AMIMOTO

horike

wokamoto

Kazuto Takeshita

webnist

gatespace

*Related posts based on categories, tags. 
*Can be replaced and sort of related posts at admin screen. 
*I can be added as an add-on another related posts algorithm.
You can make addon to use <a href="https://github.com/horike37/wp-simple-related-posts-addon-template/" rel="nofollow ugc">Addon Template</a>.
You can use JSON REST API Endpoint. Require plugin <a href="https://wordpress.org/plugins/json-rest-api/" rel="ugc">JSON REST API</a> 
 http://example.com/wp-json/posts/%post_id%/sirp_related/ 
 http://example.com/wp-json/sirp_related/%post_id%/ 
 http://example.com/wp-json/sirp_related/%post_id%/?filter[num]=5
<h4>Translators</h4>
<ul>
<li>Japanese(ja) – <a href="https://profiles.wordpress.org/horike" rel="nofollow ugc">Horike Takahiro</a></li>
<li>Thai(th_TH) – <a href="http://www.xn--12cg1cxchd0a2gzc1c5d5a.com" rel="nofollow ugc">TG Knowledge</a></li>
</ul>
<h4>Contributors</h4>
<ul>
<li>@<a href="https://profiles.wordpress.org/shinichin" rel="nofollow ugc">ShinichiN</a></li>
<li>@<a href="https://profiles.wordpress.org/webnist" rel="nofollow ugc">Webnist</a></li>
<li>@<a href="https://profiles.wordpress.org/mt8biz" rel="nofollow ugc">mt8biz</a></li>
</ul>

WP Simple Related Posts

Related Posts plugin. It's flexible and fast and simple.

Ays Pro

<h3>WordPress Related Posts plugin</h3>
<ul>
<li><a href="https://ays-pro.com/wordpress/advanced-related-posts" rel="nofollow ugc">Advanced Related Posts Premium</a></li>
</ul>
<iframe loading="lazy" title="Best Related Post Plugin - Advanced Related Post" width="750" height="422" src="https://www.youtube.com/embed/KLgGXMGSUTs?feature=oembed" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" referrerpolicy="strict-origin-when-cross-origin" allowfullscreen></iframe>
The WordPress Related Posts plugin allows you to show a related posts list on your website after a post or via a widget. The plugin offers a range of features, including custom thumbnails, meta box settings, advanced styles, and more. The plugin is an essential tool to introduce your page visitors to other related content, boost site engagement, and increase pageviews. Furthermore, it allows you to fully customize your contextual related posts list without any need for tech skills.
To activate the tool, select the post types after which you would like to display related posts. You can enable it for all post types or exclude some according to categories and IDs. Besides, you can decide the post relation type and the way of ordering the contextual related posts list. It’s possible to show excerpts of articles and give their length by characters or words. Last but not least, the plugin has a related post widget that enables you to show related articles in the most preferred place on your site. We have made sure that you can control your related post widget settings right inside the plugin for your convenience.
What makes us different? The plugin is developed by professionals who have placed special importance on quality and customer satisfaction. With its intuitive interface, you don’t have to spend time getting acquainted with the functionality. Get straight to optimizing the tool to your preference and reach expected results. Allow your visitors to discover the amazing content on your page by displaying related content under articles.
<h4>RELATED POSTS PLUGIN KEY FEATURES</h4>
<ul>
<li>Enable for all post types: Choose to display related articles regardless of the post type. By enabling the option, you cannot exclude similar articles for some categories or post IDs.</li>
<li>Enable for selected post types: It’s up to you to decide under which post types to show related content.</li>
<li>Exclude for categories and/or post IDs: Just enter the category name(s) and/or post ID(s) for exclusion. This way, you don’t display similar articles under the articles that belong to the mentioned categories or have those IDs.</li>
<li>Count of similar posts: You can decide the number of linked posts that should be displayed yourself.</li>
<li>Display on the front page: Similar articles can also be visible on the home page if you enable the corresponding option.</li>
<li>Display order results: The order of similar posts can be based on one of the following.</li>
<li>Default: Similar articles will be listed according to the filtration that has taken place. No other changes will be made.</li>
<li>Date: The ordering is done according to when articles were created. According to what you choose, the oldest or the most recent posts will appear on the list.</li>
<li>Title: Items will be shown in an ascending or descending alphabetical order.</li>
<li>Show meta box: The meta box that appeared under a post on the Dashboard will allow you to give central control to the filtration of the articles. Changes made inside a meta box are higher in priority and thus are more powerful.</li>
<li>Limit post title length: You can shorten the titles of linked posts by entering a character or word count.</li>
<li>Show post excerpt: Lastly, you can show parts of popular posts and decide their length by entering a character or word count.</li>
</ul>
<a href="https://ays-pro.com/wordpress/advanced-related-posts" rel="nofollow ugc">Pro Features</a>
<ul>
<li>Contextual related posts</li>
<li>Display related posts on popup</li>
<li>Display Custom HTML between the post title and post description</li>
<li>Add to Cart button for the WooCommerce products </li>
</ul>
<h4>SIMILAR POSTS RELEVANCE</h4>
The Related Posts plugin for WordPress does all the hard work for you. It filters articles according to some options that you have chosen inside the plugin. The options that are responsible for filtration are on the Relevance tab. Let’s get acquainted with them.
With the Order post query option, there are three ways you can filter your related content: by relevance, randomly, or by date. If chosen by relevance, the most trendy and popular posts will appear as similar articles. In the case of randomly, there’s no specified way of filtration, and random items will appear on your thumbnail-rich related posts list. Lastly, choose by date so that the system chooses the most recent posts, then proceeds with the older ones.
The Only from the same option helps you decide whether to display related posts with the same type, tag, and/or format as the current article. You can select more than one of the presented ways for the similar articles list. You can also choose to display same category posts.
With the Strongness of matching option, decide the way to filter your articles. The filtration can take place in one of the following ways. 
– Gradually weakening: The system will consider all the options that are responsible for filtering and do the work. Imagine that you have ticked all the four suboptions of the Only from the same option and have defined the count of linked posts as ten. In this case, the system will first take the items that correspond to all those four suboptions. Then, if the number of articles is not enough, the system will ignore the last suboption (post format), then the second to last one (post tag), and continue the same way unless there are ten same category posts. 
– And: It is a strict condition, and only those items that completely correspond to the chosen ways of filtration will be selected. Again, if you have ticked all the four suboptions of the Only from the same option and have also enabled the Limit to the same author option, the system will only take the articles that correspond to what you have chosen. Even if the numbers of found items (for instance, six) and the one that you predefined (ten) don’t correspond, it won’t matter: only six similar articles will be displayed. 
– Or: It is a milder condition compared with the previous ones. It suggests that the system will first take the items corresponding to the options and suboptions you have enabled. But to find the remaining number of articles, it will then consider only one suboption and start to filter articles according to it. Afterwards, the system will pick another suboption for filtration unless there are as many articles as you wish to display.
The Limit to the same author option allows you to show just the same author’s works as related.
Lastly, the Display only posts from the past option refers to those similar articles that should be displayed. You need to specify the period of time so that filtration can take place within it. For instance, if you enter 12 in the field and choose Days, only the articles that have been created during the last 12 days will be visible.
<h4>ADVANCED STYLES</h4>
The Related Posts plugin for WordPress allows you to customize the design of your related content the way you wish to. Pick one of the layouts, a text color, and thumbnail size. The thumbnail usually takes the first image inside the post. Yet, you can upload a default image for the thumbnail yourself. The plugin also enables you to write your own Custom CSS code.
<h4>RESPONSIVE DESIGN</h4>
As the WordPress Related Posts plugin supports responsive design, your thumbnail-rich related posts list can be visible on any device – mobile phones, tablets, and so forth. The layout of related content won’t suffer, as it’s adjusted to different screen sizes. As the number of mobile users is on the rise, we have made sure our plugin helps you increase pageviews and traffic.
<h4>EASY TO USE</h4>
The user interface of the plugin is very easy to navigate through, making it suitable both for beginners and developers. Besides, tooltips will come for help while trying to figure out what an option is for.
<h4>CONTACT US</h4>
Do you still have queries about the WordPress Related Posts plugin that haven’t been answered here? Feel free to contact us via the <a href="https://wordpress.org/support/plugin/advanced-related-posts/" rel="ugc">free Support Forum</a>. Our customer care specialist will give comprehensive answers to your questions within a workday.
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</ul>

Advanced Related Posts

This plugin allows you to display related posts with widget or under the post with advanced options.

WP Simple Related Posts vs Advanced Related Posts