diff --git a/app/_locales/en/messages.json b/app/_locales/en/messages.json
index 55b423894..64c95545a 100644
--- a/app/_locales/en/messages.json
+++ b/app/_locales/en/messages.json
@@ -1307,6 +1307,12 @@
     "message": "Spend limit requested by $1",
     "description": "Origin of the site requesting the spend limit"
   },
+  "spendLimitTooLarge": {
+    "message": "Spend limit too large"
+  },
+  "spendLimitInvalid": {
+    "message": "Spend limit invalid; must be a positive number"
+  },
   "switchNetworks": {
     "message": "Switch Networks"
   },
diff --git a/ui/app/components/app/modals/edit-approval-permission/edit-approval-permission.component.js b/ui/app/components/app/modals/edit-approval-permission/edit-approval-permission.component.js
index f7dad3a50..7c850279b 100644
--- a/ui/app/components/app/modals/edit-approval-permission/edit-approval-permission.component.js
+++ b/ui/app/components/app/modals/edit-approval-permission/edit-approval-permission.component.js
@@ -1,13 +1,18 @@
 import React, { PureComponent } from 'react'
 import PropTypes from 'prop-types'
+import log from 'loglevel'
 import Modal from '../../modal'
 import Identicon from '../../../ui/identicon'
 import TextField from '../../../ui/text-field'
+import { calcTokenAmount } from '../../../../helpers/utils/token-util'
 import classnames from 'classnames'
 import BigNumber from 'bignumber.js'
 
+const MAX_UNSIGNED_256_INT = new BigNumber(2).pow(256).minus(1).toString(10)
+
 export default class EditApprovalPermission extends PureComponent {
   static propTypes = {
+    decimals: PropTypes.number,
     hideModal: PropTypes.func.isRequired,
     selectedIdentity: PropTypes.object,
     tokenAmount: PropTypes.string,
@@ -15,7 +20,7 @@ export default class EditApprovalPermission extends PureComponent {
     tokenSymbol: PropTypes.string,
     tokenBalance: PropTypes.string,
     setCustomAmount: PropTypes.func,
-    origin: PropTypes.string,
+    origin: PropTypes.string.isRequired,
   }
 
   static contextTypes = {
@@ -27,7 +32,7 @@ export default class EditApprovalPermission extends PureComponent {
     selectedOptionIsUnlimited: !this.props.customTokenAmount,
   }
 
-  renderModalContent () {
+  renderModalContent (error) {
     const { t } = this.context
     const {
       hideModal,
@@ -136,7 +141,6 @@ export default class EditApprovalPermission extends PureComponent {
               <div className="edit-approval-permission__edit-section__option-input" >
                 <TextField
                   type="number"
-                  min="0"
                   placeholder={ `${Number(customTokenAmount || tokenAmount)} ${tokenSymbol}` }
                   onChange={(event) => {
                     this.setState({ customSpendLimit: event.target.value })
@@ -147,6 +151,7 @@ export default class EditApprovalPermission extends PureComponent {
                   fullWidth
                   margin="dense"
                   value={ this.state.customSpendLimit }
+                  error={error}
                 />
               </div>
             </div>
@@ -156,10 +161,44 @@ export default class EditApprovalPermission extends PureComponent {
     )
   }
 
+  validateSpendLimit () {
+    const { t } = this.context
+    const { decimals } = this.props
+    const { selectedOptionIsUnlimited, customSpendLimit } = this.state
+
+    if (selectedOptionIsUnlimited || !customSpendLimit) {
+      return
+    }
+
+    let customSpendLimitNumber
+    try {
+      customSpendLimitNumber = new BigNumber(customSpendLimit)
+    } catch (error) {
+      log.debug(`Error converting '${customSpendLimit}' to BigNumber:`, error)
+      return t('spendLimitInvalid')
+    }
+
+    if (customSpendLimitNumber.isNegative()) {
+      return t('spendLimitInvalid')
+    }
+
+    const maxTokenAmount = calcTokenAmount(MAX_UNSIGNED_256_INT, decimals)
+    if (customSpendLimitNumber.greaterThan(maxTokenAmount)) {
+      return t('spendLimitTooLarge')
+    }
+  }
+
   render () {
     const { t } = this.context
     const { setCustomAmount, hideModal, customTokenAmount } = this.props
     const { selectedOptionIsUnlimited, customSpendLimit } = this.state
+
+    const error = this.validateSpendLimit()
+    const disabled = Boolean(
+      (customSpendLimit === customTokenAmount && !selectedOptionIsUnlimited) ||
+      error
+    )
+
     return (
       <Modal
         onSubmit={() => {
@@ -170,9 +209,9 @@ export default class EditApprovalPermission extends PureComponent {
         submitType="primary"
         contentClass="edit-approval-permission-modal-content"
         containerClass="edit-approval-permission-modal-container"
-        submitDisabled={ (customSpendLimit === customTokenAmount) && !selectedOptionIsUnlimited }
+        submitDisabled={disabled}
       >
-        { this.renderModalContent() }
+        { this.renderModalContent(error) }
       </Modal>
     )
   }
diff --git a/ui/app/pages/confirm-approve/confirm-approve-content/confirm-approve-content.component.js b/ui/app/pages/confirm-approve/confirm-approve-content/confirm-approve-content.component.js
index 673bfe415..461d2e942 100644
--- a/ui/app/pages/confirm-approve/confirm-approve-content/confirm-approve-content.component.js
+++ b/ui/app/pages/confirm-approve/confirm-approve-content/confirm-approve-content.component.js
@@ -13,6 +13,7 @@ export default class ConfirmApproveContent extends Component {
   }
 
   static propTypes = {
+    decimals: PropTypes.number,
     tokenAmount: PropTypes.string,
     customTokenAmount: PropTypes.string,
     tokenSymbol: PropTypes.string,
@@ -127,6 +128,7 @@ export default class ConfirmApproveContent extends Component {
   render () {
     const { t } = this.context
     const {
+      decimals,
       siteImage,
       tokenAmount,
       customTokenAmount,
@@ -164,7 +166,15 @@ export default class ConfirmApproveContent extends Component {
         >
           <div
             className="confirm-approve-content__medium-link-text cursor-pointer"
-            onClick={() => showEditApprovalPermissionModal({ customTokenAmount, tokenAmount, tokenSymbol, setCustomAmount, tokenBalance, origin })}
+            onClick={() => showEditApprovalPermissionModal({
+              customTokenAmount,
+              decimals,
+              origin,
+              setCustomAmount,
+              tokenAmount,
+              tokenSymbol,
+              tokenBalance,
+            })}
           >
             { t('editPermission') }
           </div>
@@ -209,10 +219,12 @@ export default class ConfirmApproveContent extends Component {
                     showEdit: true,
                     onEditClick: () => showEditApprovalPermissionModal({
                       customTokenAmount,
+                      decimals,
+                      origin,
+                      setCustomAmount,
                       tokenAmount,
                       tokenSymbol,
                       tokenBalance,
-                      setCustomAmount,
                     }),
                   })}
                 </div>
diff --git a/ui/app/pages/confirm-approve/confirm-approve.component.js b/ui/app/pages/confirm-approve/confirm-approve.component.js
index bb7f6ceb2..01c93e5a4 100644
--- a/ui/app/pages/confirm-approve/confirm-approve.component.js
+++ b/ui/app/pages/confirm-approve/confirm-approve.component.js
@@ -87,6 +87,7 @@ export default class ConfirmApprove extends Component {
         title={tokensText}
         contentComponent={(
           <ConfirmApproveContent
+            decimals={decimals}
             siteImage={siteImage}
             setCustomAmount={(newAmount) => {
               this.setState({ customPermissionAmount: newAmount })
diff --git a/ui/app/pages/confirm-approve/confirm-approve.container.js b/ui/app/pages/confirm-approve/confirm-approve.container.js
index 29fec2ff3..92f0eb552 100644
--- a/ui/app/pages/confirm-approve/confirm-approve.container.js
+++ b/ui/app/pages/confirm-approve/confirm-approve.container.js
@@ -76,20 +76,22 @@ const mapDispatchToProps = (dispatch) => {
   return {
     showCustomizeGasModal: (txData) => dispatch(showModal({ name: 'CUSTOMIZE_GAS', txData })),
     showEditApprovalPermissionModal: ({
-      tokenAmount,
       customTokenAmount,
-      tokenSymbol,
-      tokenBalance,
-      setCustomAmount,
+      decimals,
       origin,
+      setCustomAmount,
+      tokenAmount,
+      tokenBalance,
+      tokenSymbol,
     }) => dispatch(showModal({
       name: 'EDIT_APPROVAL_PERMISSION',
-      tokenAmount,
       customTokenAmount,
-      tokenSymbol,
-      tokenBalance,
-      setCustomAmount,
+      decimals,
       origin,
+      setCustomAmount,
+      tokenAmount,
+      tokenBalance,
+      tokenSymbol,
     })),
   }
 }