Flexbox space-between but center if one element

For cases where you have one item in the container, you can use the :only-child pseudo-class.

Add this to your code:

.box:only-child {
  margin: 0 auto;
}

.container-box {
  width: 200px;
  display: flex;
  flex-direction: row;
  justify-content: space-between;
  background-color: red;
  margin: 50px;
}

.box {
  background-color: #9009A0;
  height: 50px;
  width: 50px;
}

.box:only-child {
  margin: 0 auto;
}

<div class="container-box">
  <div class="box"></div>
  <div class="box"></div>
</div>
<div class="container-box">
  <div class="box"></div>
</div>

In such cases, flex auto margins will override justify-content because:

§ 8.1. Aligning with auto margins

Prior to alignment via justify-content and align-self, any positive free space is distributed to auto margins in that dimension.

More about :only-child:

§ 6.6.5.10. :only-child pseudo-class

The :only-child pseudo-class represents an element that has no siblings. Same as :first-child:last-child or :nth-child(1):nth-last-child(1), but with a lower specificity.

More about flex auto margins:

• In CSS Flexbox, why are there no "justify-items" and "justify-self" properties?

Also, to spotlight some interesting flex behavior, if you were using space-around instead of space-between, you wouldn't need auto margins.

• Flex item should align left, not center, when it wraps

For info, You could also use together :first-child and :last-child if you wanted to mind about very old browsers ;)

.container-box {
  width: 200px;
  display: flex;
  flex-direction: row;
  justify-content: space-between;
  background-color: red;
  margin: 50px;
}

.box {
  background-color: #9009A0;
  height: 50px;
  width: 50px;
}

.container-box .box:first-child:last-child {
  margin: 0 auto;
}

<div class="container-box">
  <div class="box"></div>
  <div class="box"></div>
</div>
<div class="container-box">
  <div class="box"></div>
</div>