Lego Hidden Side (stylized as LEGO Hidden Side) is a discontinued Lego theme that was first introduced in August 2019. It took place in the fictional city of "Newbury" and linked Lego playsets to an app of the same name (for iOS and Android), which was used to hunt, capture and collect ghosts via augmented reality. The series came on the market in 2019 and, after the discontinued Lego Dimensions and Nexo Knights series, represented another attempt by The Lego Group to link digital and real game worlds. It was accompanied by the Lego Hidden Side webisode series and a television special, which was released on the Lego YouTube channel, and on Cartoon Network. The product line was discontinued by the end of 2020.

Lego Hidden Side was a Lego product line that was designed to provide an experience that combines both physical and digital play. This has been described by The Lego Group as "fluid play". The Hidden Side theme involved traditional brick built playsets, but combined them with augmented reality. This involved the use of a smartphone app that brought the sets to life and revealed a hidden digital world that offers various challenges. The digital world represented the "hidden side", a hidden world that is populated by virtual ghosts. The app provided the interactive aspect of the experience when a smartphone was held up to the physical sets.

The augmented reality play experience focused on the main characters Jack and Parker who begin ghost-catching in Newbury. Players explored the various haunted locations of Newbury through these characters, with clues about the city's history of paranormal activity revealed within the sets.

Lego Hidden Side began with a brief given to the company's Creative Play Lab to develop a concept for a "Play-Theme 2.0", a theme that would include augmented reality to enhance physical Lego play. The team at the Creative Play Lab came up with many concepts and tested them with children to assess their reactions. Senior Design Manager Sven Robin Kahl commented, "It was clear they needed to create a ghost theme, as that concept worked so well with the digital layer. Digital ghosts that can only be seen with the phone!"

The theme was also the result of an extensive investigation by The Lego Group into augmented reality gameplay with the use of advanced hardware. To implement the functionality, the theme required ARKit for iOS users and ARCore for Android on mobile devices. This was a capability that could be accessed by most children in the primary market. The theme was developed as a team process between the model designers and the app developers. Senior Product Lead, Murray Andrews commented that the integration of the app functionality offered certain challenges stating, "a simple example is you are holding the phone in one hand and you have to interact with the model, you have only got the other hand to do it. So they had to simplify some of the functions to be able to use one hand rather than two handed functions. That was just an interesting journey that we went on with this project, with the designers having to learn new ways of making things work."

The sets were designed to appear like existing Lego urban buildings but, when used in conjunction with the app, were interactive. The augmented reality placed a graphic overlay on the image of the sets that transformed buildings, vehicles or Lego minifigures into paranormal creations. Roberto Marchesi, lead digital creative explained, "We didn't want to have a really cool game that was working in isolation from the sets. We wanted to create a holistic experience with AR." The process of developing an augmented reality game was particularly challenging for the design team and required an investigation into what good augmented reality really meant. Marchesi explained, "When we started this process, we didn't even know what good AR play was because no one had good AR play where you go back and forth between the digital and physical interaction. There was a lot of exploration and learning based on the simple mechanics involved of looking through your phone, taking stock of what you see, and then acting accordingly."

The Lego Hidden Side theme was launched in August 2019. As part of the marketing campaign, The Lego Group engaged The Mill to create an event that was designed to create awareness of the theme. The event invited five YouTube influencers to take part in an interactive ghost hunting experience, which involved the characters from the storyline. The event was subsequently broadcast on YouTube.

According to BrickLink, The Lego Group released 32 playsets and promotional packs based on the Hidden Side theme.